Ball detent pins - name, where?
 

I found a GMPG mount for my M38A1 CDN3. It came with 2 of 3 pins. Only 1 is working properly.

These are 3/8" dia pins about 4" long. Push the thumb button and 2 ball bearings on the sides at the tip end are retracted.

What are these pins called? Where should I start looking for replacements?

Called Quick Release pins. Try McMaster-carr.

push button quick release pins
 

http://www.mcmaster.com/#90985a317/=25tthw

Yikes! Those are pricey. But they are almost correct for 1970. The trouble with McMaster Carr is, apparently they absolutely refuse to ship to Canada.

That was my experience almost a year ago, if you are lucky they may have changed policy, or they may not notice your address until the product is shipped. I hope you have better luck than I did. If so, please let us know. They do supply an amazing variety of useful hardware.

Pins
 

Spaenaur in Kitchener has these. They have all types. E mail the size you need and they will reply with pricing.
Toll free number is 1-800-265-8772.(Mike & measure before you phone)
Their catalogue is 2 1/2 inches thick and invaluable.

Peter Simundson

Their on line catalogue is .pdf pages and takes forever to load, but yeah, they've got it all too. Also looks like some lower spec' models at much less painful prices. But the C$ is improving on the US$ so I might have to think about importing other parts.