I've done the convertible thing and it just isn't for me.
I had a S&W 48 with factory fitted second cylinder back in the 80s, along with Dan Wesson 722 and 715, both with a handful of different barrel lengths.
I've just found over the years, that I prefer a whole gun to a conversion.
My Advantage Arms Glock .22 conversion has it's own frame, as do both of my Kimber conversions (.17HM2 and .22LR).
"Apparently not, but like me, there are more than 200 people that want to know about your inquiry.
You got me wondering now, or I would not have writen nothing."
And it took 22 days to make you wonder but not ask?
"aarondhgraham" and "9x19", not about the "combo" but both of you did provide the information I was looking for, thank you!
"...the convertible thing and it just isn't for me."
Not sure either but I've had very good luck and a lot of fun with my Colt "conversion unit" and AR-15 "conversion unit".
Already have a pair of S&W 617 .22 revolvers so I'm looking at the "combo" more as a .22 Mag gun with a .22 cylinder for occasional use.