Thanks for the input, guys. Fastbolt - I read through your instructions from older thread--the updated pictures help. I think I understand everything.

I've got some work to finish today (i.e., the kind that actually pays me) but I'm tempted to try cannibalizing the intact part from my older 4516 and installing it in this new gun. Between the instructions here, and the videos, I should have everything I need to know. Only thing I don't have is a rubber/non-marring mallet to dislodge the sideplate from sear pin. Hopefully Ace would have something like that.

I would of course install the sideplate I just ordered in the old gun, once it arrives. But if all goes well, maybe I can get the new gun out to the range tomorrow after all.

Anyway.. thanks again for all the advice. I will assess the situation in more detail later to decide if it's a challenge I want to undertake.