Jim, your idea to chase the military line is starting to bear fruit. Nothing conclusive yet, but I think I'm on the right track.
C&P from www.subguns.com:
"...The earliest guns used an inertial weight to unlock the slide lock. My oldest one has a round button on the left side of the receiver to do this..." -John Browning's original design!
"...while a slightly newer one I had used a small lever jutting out from the bottom of the receiver, but it was still attached to an inertial system..." -Just like mine!
"...The later(after mabey 1906/08?) guns use a modern system where a spring extension rides in a slot on the side of the hammer. The falling of the hammer allows the spring to move the slide lock out of engagement with the slide, unlocking the gun for a quick reload..." -Posted By: Chuck Madurski (credit where credit's due)