I'm not sure what you mean by cable release socket, but if it's part of the shutter, you might call Carol Flutot at Flutot's camera repair. she specializes in shutters and might have one off a junker shutter. If not her, then try Paul Ebel in Spring Valley MN. He also specializes in shutter repairs.
I find the term quite apt, would not know a better one. The shutter has a kind of cubical piece of metal screwed to its side. It got a hole with that standardized conical thread to accept the male thread of a release cable.
In case it is about a defect female thread, a good repairer should be able to re-cut it.
The Copal 3 does not have a block on the side like a Copal 0 or 1. It has a socket (screwed) into the shutter. Seeing as Copal shutters are no longer manufactured, these parts can be really hard to find (and expensive.) I had to replace one the threaded blocks for a Copal 1 and was shocked at how expensive it was. I suspect the Copal 3 version is scarce and expensive as well.
Oops., I looked at the wrong photo. (I have not yet come across this shutter, we got the german ones here.)
Anyway, I still do not see the problem if the issue is that thread.
Rory,
Contact cusotmer service atDead Link Removed. You can email them at customerservice@criscam.com
Explain that you need/want to purchase repair parts for a copal #3 shutter. They'll send you a PDF showing an exploded view /part list. You email back what part you need. they send you a quote...you pay, they mail the parts. Easy.