Welcome to the forum Henry, good to have you as a member. The proof mark you referred to on your breech block is a standing eagle as you said and the "P" on your frame and barrel are part of the serial number, 1570P. As Lugerdoc pointed out it appears that you are missing the hold open and you are in luck as Lugerdoc usually has them in stock. Good luck to you and thank you for your military service.
Lon