All was well until it came time to remove the link pin stop screw. This is a small screw that simply keeps the link pin from walking sideways out of the link & receiver. It's threaded near its head, but not toward its tip.
It shouldn't take much torque to remove this screw, but it was very stubborn when I tried to remove it. I applied a little elbow grease, and WHAM! the head broke right off the screw. (Note: I didn't actually hear a "Wham," but I felt one in my guts.)
Turns out, there was a concave worn spot on the unthreaded tip of the screw, where the link pin had been riding against it. The pin was lying against that concave spot when I was trying to remove the screw, so the screw couldn't turn. Instead, it went ahead and broke. Let this be a lesson to you - make sure all parts are aligned when you're working on a gun.
The photo shows the bottom of the rifle's receiver, with the broken screw head lying next to the rest of the screw. If you look closely you can see that the end of the link pin is sticking out of one side of the receiver a little (bottom side in the photo). That was enough to cause the worn place on the screw to bind on the pin.
If you look to the right of the screw, you can see a tiny bit of original factory bluing remaining on the front end of the lever.
I found a new screw at Numrich Gun Parts Corp, and ordered it. The tiny sucker cost $1.85, plus $5.95 to ship it. That's almost eight bucks for a screw that's just over 1/4" long! Ouch.
In case you're wondering, I removed most of the gun's serial number from this photo.
Next, I had to remove the broken screw.