Kevin,
Honestly I’m not seeing a head stamp. To me, the base looks similar to the base on the Frankfurt Arsenal blanks. That said, there is an almost exact match in Roundball to Rimfire-, the Crittenden & Tibbles with no head stamp. Weight 471, OAL 1.61, diameter .498.
The only other that is close is a head stamped SAW weight 472, OAL 1.61 and diameter .500. There is a SAW without head stamp listed with slightly different measurements but within expected tolerances.
Unfortunately I don’t have all the different Spencer types in my collection, so I can’t match the base exactly and the books don’t picture the bases of the non marked cartridges.
Mike