Material that's in the public domain remains in the public domain forever. Nobody can attack it and nobody can defend it. You can slap a bogus copyright notice on a public domain work, but that does not change its public domain status.
<warning: personal interest coming>
Do you happen to know if this is true in Canada? I ask because the Gov't of Quebec has several photos on one of their archive websites that are definitely old enough to be P.D. but which they claim to have copyright over. I'm wondering if I could get away with using them without asking.
M