Here's a link to a graphic on my site of an actual 15GP22 gun that was used years ago in an attempt to rebuild the 15G. We now have new stems as you know, but back then they were not available and so this was someones way of trying to make do. This gun is welded onto a 14AH button, the base for the 21AXP22. No one knows if this approach was ever actually tried, but it seems unlikely. As you can see, this particular guy has been in the air for many years now.
http://home.att.net/~pldexnis/15GP22...em8bitgrey.jpg