This turned up on my desk today for some reason. The markings on it are SPU4. It is have round with an indent on top. I did a search and found nothing. Any idea what it is?

My guess is an ancient transistor from a project I never built.

 

Looks more like a penny to me.

 

Any idea what it is? My guess is an ancient transistor from a project I never built.

That would be my guess, also; some of the first silicon transistors I acquired, ca. 1965, were 2N2923's and they were in that funny package.

EDIT: turns out that package is the TO-98. Here's a copy of an old GE datasheet for the 2N2923: