John,
Here are samples from a 1914 Erfurt Artillery. The breechblock proof is inset into this composite photograph (I enhanced it a bit, the stamp is very weak). It is indeed the same Arsenal Acceptance Eagle found on the chamber and the barrel, as illustrated. Also included is a closeup of the top of the extractor.
Also included is a scan of this mark as illustrated in Costanzo's "World of Lugers Proof Marks", p.66.
Hope this helps.
--Dwight
PS Ron Wood, you were right (re: Costanzo), I've -already- consulted it a jillion times!