M12 Win . NOS barrel .

I'd have sworn there was a shotguns forum here about ........
So here's my situation I have the extremely rare opportunity to fit a October/November 1924 Winchester M12 with a NOS barrel already fitted with the extension , at a bargain basement price compared to current values.

Why well at some point 1/1/1925 and June 2008 somebody stuck a wad and got a ring right on the front edge of the barrel/mag tube clamp and the bead was replaced and drilled clear through as well as the choke having been lapped out to Imp/cyl and or just cut off . In any case to be really safe and me comfortable sending it on to my grands it needs to be done and the opportunity came to do it .

Long ago Dad and I talked about setting up the headspace , lapping rings , adjusting fit and all of those virtues of the glorious Model 12 . Unfortunately Dad's gone now and the memory of the things we talked of 30 yrs ago waiting out a blizzard in a quakie run at 7,000 ft during a deer hunt isn't as sharp as it was 29 yrs ago ....... I know that there is a chance that it will drop in close enough . I know that it's as easy making a 15° adjustment of the interrupted thread extension which requires just a little more than a screwdriver and a feeler gauge . Obviously headspacing is in order but that most likely will be a product of lapping to fit .

So I guess what I need is someone that can refresh the details and pitfalls before the new assy arrives tomorrow morning and be sort of available if I can't make "all the little springs and balls go back in the holes " .