Use painters green tape or even electrical tape to protect the receiver from marring, especially when putting the bolt catch pin in. Take your time and don't do it while drinking - although you'll want to start as soon as you're done!
Nice looking receivers. What did the engraving run you altogether? (you can pm me if you don't want to post that here)
ETA: Do it over a tile floor if possible, not a carpeted floor. Ask me how I know.
(I always buy extra detents & springs JIC)