A personal foible of mine is that I like socket head screws on all my guns. I just took a glance at my 631, and sure enough - at some point in the past I had replaced that rear mounting screw with a black alloy socket head cap screw. That means I got the screw at the local hardware store. Take yours to the store and I'll bet you can match it. I don't mean a home improvement store, where they only have a few SAE sizes in poly bags. I mean a real hardware store, where they have little drawers full of stuff, both SAE and metric.
That screw is the same for both the 630 and 631.