You're partly right. In that time period you not only had to register, you also had to make a good faith effort to add copyright notices to the works you wanted

You're partly right. In that time period you not only had to register, you also had to make a good faith effort to add copyright notices to the works you wanted protected. The copyright registration database is online [0]. He only registered a few of his art works, and these digital images are not among them. edit: his estate registered a bunch of stuff after he died, but registration doesn't bring things back out of the public domain for pre-1977 works, or things beyond the 5-year threshold for works 1977-1989 [0] https://cocatalog.loc.gov https://cocatalog.loc.gov