Mike,
I took the breech block apart and see the end of the screw is formed to guide the firing pin via the slot. I am beginning to think the presence of a screw head or the use of a screw with the head peined and dressed off is a change made along the production history. If one tried to remove the screw prior to removing the firing pin, the guide nub would be broken off or damaged. Simplest way to avoid the problem from Fraser's point of view would be to dress off the screw head.
Thanks,
Jim