I think this mold may have been for pewter instead of silver. Think
bullet molds. Considering its age and value I would not want to risk
molten silver. Also there is not enough mass there for the heat to
go somewhere. Is it a two part mold? I would do a wax cast then cast
the wax to get a metal master and make a new rubber mold. You will
have some shrinkage from the original but no chance of damaging it.
I have had brass molds for casting plaster ornaments for
architectural work. Several of the books in my library show molds
for tea pot handles and spout parts from that time period. Those
molds were in cast iron. Something to think about.
Bill Churlik
www.earthspeakarts.com