🚀 go-pugleaf

I am posting this on behalf of a friend.  He has an HP412A precision
DC voltmeter-ohmmeter-ammeter, circa late 1950s.  In the course of
repairing it, he came across an unusual component.....it is in a gold
metal can (transistor size), and has two leads coming out.  It also has
a small indentation stamped in the top, opposite from which are painted
yellow and violet dots.  When removed from the circuit, and tested on
a VOM, it appears to be a diode.  There are no other identifying features.

Can someone advise as to exactly what this part is?  Does anyone have
a schematic or service manual for this antique?

Replies via email to sender, or to MRB @ PSUECL (via BITNET), and I
will watch this newsgroup for followups.

Thanks in advance for your help.

WA3ZXO