A tired old Soldier…

[Don M]

While I can't be sure, it does look as if the narrow strip of shelf outside rivet hole has corroded away. I think the odds are better than 99% that it once had a sear safety that was removed and that the side plate was replaced with one that did not have a slot milled in the top for the sear safety.

There is also clearly a hole drilled in the frame above the trigger that was required for the mag safety mechanism.

It is probable that this pistol was transferred from the police to the military where the sear and mag safeties were removed.