Here is a nickel Model 15 flanked by a model 12 and 66

Not really sure what niche the Model 15 hit with its Square butt, 2" heavy barrel and target sites.
Probably would have been better suited as a 3" or roundbutt .

It occurred to me recently the Model 15's frame is in reality a Model 14 frame with a snub 66 long rear site and 66 heavy barrel profile.