Copyright claims from museum and libraries mean absolutely nothing in the general case. Their websites tend to tag copyright claims on everything and anything, including public domain work. One should not take it as these claims have any sort of substance -- nor that they do not: you just can't tell.
From the point of view of museums, it costs nothing to claim ownership of
public domain material. Noone will sue them for this, and it saves them the trouble of sorting which work to tag with what. Of course this hinders diffusion of culture and knowledge among the public, but museums usually intend to sell tickets, postcards and licences. Culture is a pretext and cover -- pretty much like information is a pretext to newspapers and television to feed the public with insignificant and sensationalist anecdotes of dubious merit and truth value, in order to sell advertisment.
The only thing that we can say with confidance is that if law and museum copyright notices contradict each other, law wins. -- Rama