Quoted from https://fairuse.stanford.edu/overvi...ain/welcome/
Under copyright laws that were in effect before 1978, a work that was published without copyright notice fell into the public domain. If the work did not include the word "Copyright" or a © (a "c" in a circle) and the name of the copyright owner, the work would enter the public domain. This rule was repealed; copyright notice is not required for works first published after March 1, 1989 (although works first published prior to that date must still include notice). Just because you find a copy of a book without a copyright notice doesn't mean that the work is in the public domain. It's possible that the copy you are viewing is unauthorized or that the notice has only been removed from a very small number of copies, both of which are excusable. It is also possible that the author followed a copyright law procedure for correcting the error. And, if you're using text from a journal, anthology, newsletter, or magazine published before March 1, 1989, check to see if there is a copyright notice either for the individual article or for the whole publication. Either type of notice will prevent the work from falling into the public domain.
So, my understanding (I'm not a lawyer) is that they are not required to have the "©" on every single trading stamp that was printed prior to 1978 -- only somewhere on the full roll/sheet. Indeed, USPS had it printed on the pane selvedge.
As Codehappy also pointed out, trademark protection may also need to be considered.