Socket is Mac and there is more than one number on it. When I went to the Mac web site none of the numbers came up using their search function, what is stamped is (I think) PN SC130, also E2, and a small 01. Best bet would be SC130, the others may be date of manufacture and maybe the plant?
Someplace in the garage is an old mac catalog I'll take a gander in there and report back.
Edit:
From a 1990 Mac catalog it's PN SC130 "Eaton Tamper-Proof Socket" for a #2 Eaton, price back then was $7.15 (now it's probably $27.15).