If you look at the indicia for the comics in the 1960s and early 1970s, it says "published by National Periodical Publications."

But all those comics still have a prominent circle with a "DC" in the corner on the front cover.

But when I was a kid buying comics in the 1970s, I don't remember anybody ever calling it "National."

Initials always seem cooler than full names...ie Adam Jenson = AJ or Mary Jane Parker = MJ (and the last lettering being a J is just automatically that much better...)

It's actually more complicated than that. The company published under multiple names prior to the merger with All-American in '48, beginning with National Allied Publishing and including Nicholson Publishing Co., Detective Comics, Inc., Superman, Inc., and World's Best Comics Co. With the merger, it became National Comics Publishing, Inc., which didn't become National Periodical Publishing, Inc. until 1961.

Cei-U!
I summon the tangled web!

DC indicia publishers from 1935-up:
National Allied Publications, Inc.
National Allied Newspaper Syndicate, Inc.
More Fun Magazine, Inc.
More Fun, Inc.
Nicholson Publishing Co., Inc.
Detective Comics, Inc.
A.I. Menin, Rec. Nicholson Publishing Co., Inc.
All-American Comics, Inc.
Picture Comics, Inc.
Picture Publications, Inc.
Superman, Inc.
J.R. Publishing Company
World's Best Comics Company
Jolaine Publications, Inc.
Wonder Woman Publishing Company, Inc.
Trafalgar Publications, Inc.
M.C. Gaines
Gainlee Publishing Co.
Tilsam Publications, Inc.
Wm. H. Wise & Company
National Comics Publications, Inc.
Signal Publishing Company
Beverly Publishing Company
Arleigh Publishing Company/Corporation
National Periodical Publications, Inc.
Hampshire Distributors Ltd.
DC Comics Inc.

Marvel does have more.

Marvel indicia publisher from 1939-up:
Timely Publications
Timely Comics, Inc.
U.S.A. Comic Magazine Corp.
U.S.A. Comic Corp.
Complete Photo Story Corp.
Select Publications, Inc.
Manvis Publications, Inc.
Snap Publishing Co., Inc.
Newsstand Publications, Inc.
Jest Publishing Co., Inc.
Current Detective Stories, Inc.
Western Fiction Publishing Co., Inc.
Daring Comics, Inc.
Young Allies, Inc.
Fantasy Comics, Incorporated
Medalion Publishing Corp.
Marvel Comics, Inc.
Chipiden Publishing Corp.
Animirth Comics, Inc.
Atlas News Company, Inc.
Euclid Publishing Company, Inc.
Hercules Publishing Corporation
Miss America Publishing Corporation
Margood Publishing Corporation
Comedy Publications, Inc.
Gem Publications, Inc.
London Publishing Corporation
Bard Publishing Corporation
Sphere Publications, Inc.
Zenith Publishing Corp.
Non-Pareil Publishing Corp.
Red Circle Magazines, Inc.
20th Century Comic Corp.
Cornell Publishing Corp.
Official Comics, Inc.
Crime Bureau Stories, Inc.
Exclusive Detective Stories, Inc.
Postal Publications, Inc.
Crime Files, Inc.
Comic Combine Corporation
Marjean Magazine Corp.
Prime Publications, Inc.
Leading Comic Corp.
Interstate Publishing Corp.
Classic Detective Stories, Inc.
Broadcast Features Publications, Inc.
Eye Publishing Corp.
Mutual Magazine Corp.
Classic Syndicate, Inc.
Sports Action, Inc.
Foto Parade, Inc.
Leading Magazine Corp.
Lion Books, Inc.
Male Publishing Corp.
Canam Publishers Sales Corp.
Official Magazine Corp.
Olympia Publications, Inc.
Warwick Publications, Inc.
Jaygee Publications, Inc.
Emgee Publications, Inc.
Vista Publications Inc.
Classic Publishing Corp.
Atlas Magazines, Inc.
Zenith Books, Inc.
Perfect Film & Chemical Corp.
Magazine Management Co., Inc.
Marvel Comics Group
Marvel Comics