The Manhattan project is something of a hobby of mine. I don’t
recognize those marks, but the only thing I can think they might
have been using a Pt crucible for is some of the early micro
chemical work on U235 and Plutonium. Neither of these are things you
really want to be dealing with, even in residue.
Step 1: wrap it in lead foil, and leave it there until you can find
someone with a working geiger counter. Lacking lead foil, dunk it in
a fish tank. Water’s a good moderator.
If memory serves, plutonium is mostly an alpha emitter, and that’s
easily stopped. I’d suggest talking to the local university’s chem
department. Explain what you’ve got, and that you’d very much like
to have someone with a calibrated geiger counter check the thing
over. Once you’re sure it’s cold, then you can play with it till
the cows come home. I know Los Alamos has a historian, I’d suggest
contacting him, to see if (A) he knows what it is, and (B) if it’s
worth anything to them for historical value.