OK, so we're talking at least Cogswell and Harrison and Parker-Hale, and I was thinking I may have seen some mention somewhere of BSA doing some. Did most sleeve the barrel and chambers of the existing cylinder or were some replacements made up from scratch?

It just occurred to me that the extractor would have been a challenge... you would either have to build up the original to reach the smaller 22 LR case rim or make a complete new unit with its relatively complex system of notches for the hand to engage and allow the movement needed to bring up the next round. Neither of these solutions strikes me a particularly simple.

Of course there's also the matter of adjustable sights of one sort or another that got added on to some/most examples. Were these available from some entity in England or did the various manufacturers each build their own variety?

I had taken for granted that this could have been sort of a postwar cottage industry. Now I'm beginning to see why it would have been a factory undertaking rather than a small shop, one-off type of endeavor. I still find the whole topic to be interesting though, having had a couple of small experiences in caliber changes. Thanks all (especially Jim) for your forbearance with my ramblings and speculations!

Froggie