I Dood It!!
You're getting on in years if you can remember the cartoon character with that line.
I Dremeled about 5/8" of a 1/4 inch rod, and used it to depress the plunger as I tightened the vise. Luckily the retaining pin came out with just a push from a pin punch. As an aside, my mainspring housing has a little rod inside the hole for the spring to go around, probably to act as a spring guide. Never seen that before.
Assembly was something else: my little Sears vise was only about two turns from the screw from coming off when I got the spring, plunger and depressor between the jaws. They went together OK. Then the retaining pin decided it would be cranky and wouldn't go in as I pushed. I tapped on it with a light hammer; there was a 'sprong' and something hit me on the chest. I thought: "Horse Hockey and Holy Water; I've lost the whole mess; retaining pin, plunger and spring". But when I looked down, the plunger was in place and the retaining pin was all the way in. The only thing missing was the depressor, which vanished into a corner that was difficult to get at and full of stuff difficult to move. But that 1/4 inch rod will yield many depressors.
There ought to be a jig that would make the job easier. I thought of modifying a C clamp, with the screw as the depressor and the base machined to hold the mainspring houing at the right angle, with a groove for the lanyard loop if present. I'm nowhere near able to make such a contraption but if somebody with a machine shop does make one, I'll sure buy a copy.
At least in my 45, the reduced power spring (18 lbs, 20 is the factory spring) was able to light off a variety of different brands of primers. I had no trouble with ignition and the hammer was a lot easier to cock.