I went ahead and made a trip spring from an old dull hacksaw blade. Grind the teeth off with a dremmel, enlarge the hole, grind the other side to make the hole centered, then profile it to fit the space.
Wouldn't ya know, the old firing pin with broken head was just the right size to make a trip pin out of. File the tip square, and use a sharpie with it sitting in the hole to mark where the notch for spring needs to be. I use a 60deg. needle file to start the groove, and finish with a hacksaw. Light pressure with the old dull blade off the handle for the last few strokes smoothes out the notch.
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The trip spring needs to have a bend in it to engage the block under the barrels, so start with it slightly long, clamped in the vise, and gently bend it. Get it right the first time so you don't work harden the spring steel too much. 2 bends...one down and the second one to make tip parallel with the block as it comes down.
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Finish the pin with a notch in the top to make it easy to align during assembly.
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I think my spring is a bit thin. It doesn't really work with the trigger plate off - it's not thick enough for the screw to pull it tight and flat against the frame. However, it works as it should with the trigger plate on.
The gun is now complete and all parts accounted for. I think next I'll repair the stock, followed by dent raising, cleaning up all the screw heads and finally refinishing. I have plenty of file work do on the bottom of the receiver, but I don't think I will need to remove or polish the barrels. An iteration or two of rust bluing followed by a light polish with some rottenstone should bring it back up to snuff.
I think I might give this stuff a try on the receiver to re-do the color case hardening -
https://steelfxpatinas.com/product-c...-starter-kits/. In my experience, original color case finish is about as durable/delicate as cold bluing anyway, So there is no loss in my book by trying it.