Ornamental or mechanical piece of ?

[stringplucker]

Not sure at all what this item may be. Non magnetic and silvery where I nicked it. Your guess is as good as mine.

 

[creskol]

Looks like the remains of a fancy 1920'ish radiator cap, maybe something along these lines:

 

[Breezie]

Yeppers, Creskol is on the money; nice ID! Breezie

 

[stringplucker]

Makes sense to me. I would have never guessed. The absence of any threads on the inside ring had me wondering. Another hunk of metal to add to my collection. Thanks.