The knurled rod is indeed for extracting the casings after firing.
The gunsmith who is selling this, is at the other end of the Netherlands.
And although Netherlands is small, it is still almost a 3 hour drive. I am not completely sure about how the barrel is fixed.
I think you could be right, as far as I know it works somewhat similar to a lathe-collet?

Now I'm off to find that post on the Model 39!

Maybe I shoud add this is a USED set?

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anton