An Untold Story of Superman's Childhood
"Superbaby's Lost World"
by Leo Dorfman (as Geoff Brown), Curt Swan, and Murphy Anderson
Action Comics 399, April 1971

In otherwords, Superbaby's back! Although, since we're going backwards in this part, here's a Superbaby story that was published prior to the ones covered earlier in this thread. 

Synopsis:

OK, now here's a grabbing opening sequence: Superbaby defends the Kents from a tyrannosaurus in the jungle! As the caption asks: "Are they on some weird trip through time? Are they visiting another planet? Why is the tot of steel slugging it out with a giant reptile?"Sorry to disappoint anyone, but the Kents are just taking a tram ride through the Wonderland Park Prehistoric Trail, and baby Clark has just taken a robotic dinosaur out of commission. For a kid with a "super brain", he sure seems to have a lot of trouble following his parents' never-ceasing instructions not to use his powers in public! He responds to the latest ultimatum--"behave or this is the last ride we take!"--with a pacifying "Me be good boy, Mommy!"Unbeknownst to anyone, arriving at the park are Connie and Hyde (the county's top jewel thieves--and yes, in whatever presumably rural county Smallville was in, there was plenty of competition among jewel thieves!).  The lovebird larcenists are toting a cool million in a straw basket, which that ditch in a garbage can when the police trace them to the park. 

Clark, in an effort to prove he's a good boy, retrieves the loot and returns it to the thieves, who find themselves the temporary guardians of Superbaby, who has now lost track of his parents. They figure that with a babe in tow, the cops won't look twice at them. (Ma and Pa Kent, meanwhile, aren't too concerned, since they know Clark is not in any danger). 
Connie and Hyde take Clark on the Jungle Land boat trip as a way to "lose the fuzz" and distract the squalling Superbaby:




As usual, tending to baby Clark is not as easy as it appears: he sinks the boat trying to bring the robotic hippo on board, revealing his powers to the confused criminals when he rescues them from drowning. Next he flies them over the artificial volcano to dry their clothes, where they're overcome by smoke, which leads Clark to blow them off the fake mountain.
Their caught by a convenient tree, but they've dropped the loot, and the arriving cops make their arrest--but Hyde is eager to go with them:

 

As always, no one believes the romantic rogues' crazy story, except for Jonathan and Martha, witnessing the arrest. They find Clark just where Connie said he was, on the volcano, where he's destroying more park property, an animatronic gorilla. 

At home, he hugs his teddybear, secure in knowing he proved he was a very good boy after helping: "...nice lady and man be very good to me, so me be nice to them..."

Thoughts:

So, Smallville has a nearby Disneyland-level theme park? 

Ma and Pa Kent here are depicted as an elderly couple, with grey hair. Over in Superboy around this time, they had been "de-aged" by approximately 20 years in Superboy #145, 1968. Since they were exposed to the youth serum when Clark was a teen, it makes sense that they would appear older...err...when they were younger...right? But then, as we've seen, the Superbaby stories published after this one also featured younger versions of the Kents, so I think this story has the more appropriate depiction. Superbaby's parents should look younger than the Kents did when they were rejuvenated, but only by 10-13 years, not 20, and it had historically been established that they were already fairly old when they adopted Clark. 
The Kents don't seem very concerned about all the damage Clark does here--those realistic robots can't be cheap! And once again, despite their clearly justified fears about Clark openly displaying his powers, they don't get bent out of shape when Clark goes missing.

The title is kind of a cheat, implying some Conan Doyle-style dinosaur action, justified only by the two opening panels of the story. The baby talk was as inauthentic as always, but that's half the charm of these stories.  Over in the lead, the cover was teasing horror, with a pair of Superman corpses in a mausoleum, so we have the same situation noted in the Superboy stories covered previously: whimsical sitcom-influenced Superbaby stores backing up a more sinister lead.

Of note: we see some African Americans as "extras" in the amusement park, still kind of uncommon in the early 70s.

Untold Tales of the Fortress  
"Spawn of the Unknown"
by Leo Dorfman (as Geoff Brown), Curt Swan, and Murphy Anderson
Action Comics 398, March 1971

Synopsis:
The story opens with a visually interesting scene, as Superman answers the call of the game keeper Ituru, in an extinct volcano where beasts are being turned into plants. 

According to Ituru, this is the result of famous botanist Prof. Bruno's synthetic seeds, which grown fantastic vegetation, including carnivorous plants. Among his other inventions is a plant hormone that brings dead wood back to life. 

Ituru is superstitious: "Be careful, professor! You're tampering with secrets no man should meddle with!"

Weeks later, Ituru investigates an explosion in the volcano, and finds that not only is the camp destroyed, but Bruno's hormone has scattered, transforming animals into plants. Investigating, Superman is horrrified at his next discovery:



Before he can do anything to aid Kara, Superman is summoned to Egypt to intercept tomb raiders. The tomb's statue of Anubis has a terrifying warning to the robbers:



Well, actually, that was Superman in disguise, demonstrating a use of x-ray vision I don't think we've ever been shown before (or since). Satisfied that his scare tactics achieved something that will deter future raiders even better than an arrest would have, Superman returns to his cousin back at the volcano. 

Attempting to uproot Kara and take her to the Fortress, the living Supergirl/plant creature coils itself around him, and he responds with gruesome results:



Superman bemoans the fact that now, even if he can return her to human form, she'll be an amputee, and begs forgiveness.

Hey, good news! Supergirl arrives, all in one piece and fully human. The plant creatures were only imitations of the nearest animals, the result of Bruno's synthetic seeds scattered everywhere. As proof, Supergirl directs his attention to a hideous imitation Superman that has sprouted since his arrival!

But all life is sacred to the heroes, who detach the infected area and fly it to another world where it can flourish harmlessly...all except for their own plantlife parallels, who are potted for posterity in the Fortress trophy room!

Thoughts:

Here's another example of DC's use of horrific story elements to appeal to a changing audience. This is some pretty nightmarish imagery, and I'm struck by the similarities to Alex Garland's 2018 film Annihilation (moreso than the Jeff Vandermeer novel on which it was based). The film had plant-based doppelgangers of humans and animals generated by the mysterious forces at work in an isolated area, just like here!




Even the diversion to the tomb raiders served only to indulge in some more shocking imagery, at the expense of adding another, rather ridiculous super-ability, making the effects of his x-ray vision visible to others. I guess that might come in handy at disaster scenes, but I think he generally stuck with verbal diagnostic assistance from then on: "Doctor, this man has a broken rib!"

We see some more African American presence. Ituru is superstitious, not in the primitive "native" superstition we used to see so often, but in a more civilized cliche. Bruno's fate is ignored; presumably, he blew himself up with his biochemical experimentation. 

As is so often the case in these back-ups, it's arguable whether the story genuinely deserves to be called an "untold tale of the fortress." As a reader, I'd be anticipating either a story that took place primarily in the fortress or which explained or elaborated on some of the things we had previously seen in shots of the fortress. Just telling a story and then saying "we put the souvenirs of this adventure in the fortress" doesn't quite cut it for me. 

ps You'd be amazed at how many collections of O. Henry stories leave that one out!