Yes, of course DC's SHAZAM! did not appear until 1973... but that's irrelevant, since if DC (in conjunction with the then-owners of the original Captain Marvel, Fawcett) were going to sue Marvel, it wouldn't have been over the use of the *name* Captain Marvel, since the trademark on the original had lapsed, and Marvel picked it up and re-trademarked it, perfectly legal. What they -in theory- *could* have sued over was the similarity of the Marvel Captain Mar-Vell to both Superman (alien with super-strength who can fly and wears a red-and-blue costume) AND to Fawcett's Captain Marvel (the whole alter-identity switch with a teenage boy, plus villains named Dr. Mynde, Dr. Savannah, and Dr. Minerva... it was a bit much). And of course there would be no point in suing Marvel *before* 1973 -- but guess what? Even though Marvel wasn't making much money off CAPTAIN MARVEL, if DC and Fawcett had teamed up to get a court-ruled "cease and desist" order against Marvel, they wouldn't win any damages... but they could have gotten the trademark back again!