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.
I'm like you and replace with allen head screws as they're just easier to get on and off and not worry about stripping a phillips head.