[318is_Parabellum]

Yes indeed it is a long frame, thank you for pointing that out! Makes the pistol that much more interesting, indeed!

Another difference I'd noted was the reversal of the safety...the "gesichert" had been milled out from it's original "down" position, and "gesichert" milled into the "up" position.

I had wondered about the meaning of the crown proofs with no script D or other proofs below them... I am not as well-studied on WW1 P. 08's. Is the crown proof on the breech block a commercial one?

Fascinating to find out that the frame was an arsenal replacement part originally... What implications does that fact have with respect to the chamber date and/or serial number, I wonder?

I did recognize the Mauser "42" code on the re-arsenal barrel and grips. That, along with the E/655 proofs helped me to ascertain an approximate date of the rebuild, and possibly why...