I believe this device is a GPD from the AVANTEK range in the late 1970s and early 80s.?
See datasheet at: ?http://www.rf-microwave.com/en/shop/0/393-high-performances-professional-mmic/4168-GPD-401.html
There were three and four pin devices made as the datasheet shows. Does this help in identifying the device?
Pete
G4GJL
Pete_G4GJL