Dave - I would shy away from saying I've seen it in what is now Lehigh Co.; the pieces I've seen outside of the upper Susquehanna region were all attributable to Berks Co (various of the Peter Angstadts arond Kutztown/south of Kutztown, and some funky pieces probably from up around Albany and Greenwich twp.).
I have seen a good number of Baum rifles w/ pipes made like this. Signed rifles. I wouldn't say all of them and frankly I've kind of filed the design away in the back of my mind as a 'typical' upper Susq. trait. although you won;t see it on every rifle of the area obviously.
Very definitely need to form the tailpiece before riveting it. Rolling down the forward portion - the portion which attaches to the pipe - is akin to forming a brass 1-pc. nosecap. This creates the slight 'step' in the finished product. You'll need to clean up w/ a round file afterwards to carefully fit it to the pipe. Copper works well for rivets here as it is even softer than brass so can be riveted easily w/o too much hammering (and thus deforming the brass). After you roll your pipe, make the tailpiece and orient the two pieces, drill through w. a tiny drill then disassemble - peel the pipe back open slightly so you can get a larger drill bit at the inside of the hole to create a tiny countersink on the inside of the pipe. Make a rivet in a nail/rivet header so you already have a peined head on one end - this goes inside the pipe - put a mandril in the pipe to hold it in place and then reinstall the tailpiece and w/ the pipe clamped in the vice w. mandril in it, hold the tailpiece in place and very lightly pein the outside of the rivet. Look at it closely in profile, make sure angles are what you want, then solder.
