IIRC, and I think I do on this matter...
You will send the gun and barrel to NEF for fitting...
I do not know for sure on this but my fear would be...
On a used one if it was fitted to a gun and lost more material at hinge and breech than your gun needs, it might be "over cut" or what ever the correct term would be...
And to top it off, not all NEF barrels can be put on all frames...
In a nutshell, I would make contact with NEF and ask them directly before buying...
This link is to an NEF specific section of a different forum, I admit most of my knowledge came from reading these folks exploits adventures and "builds" of the NEF lineup...
http://www.boartuffoutdoors.com/cgi-...nglandfirearms
Brent