Here again is the lefty B-M I got from MikeFinn. That cut down 6" Colt N.S. fits but the trigger guard bangs against the spring. Mike had a 6" 27 in it and my 25-5 fits perfectly. This barrel extention is exactly for a 6" n-frame and was done long ago.
My question to all you experts is this. What N-frame S&W with a 6" barrel was this holster modified for? Except for a 6" Reg. Magnum .357 or a Model 27, what N-frame was 6", not 6 1/2" like most of the classics were? This is an old extension so consider the 1950's the max. time period.

N-frame is N-frame is N-frame....
but considering the potential era of use it would be any 6" N-frame from the TL on up?

But wasn't the TL a 6 1/2" barrel only? I'm just wondering what 6" N-frames besides the Registered magnum and pre-27 came with a 6" barrel, and I can't think of any. Pp to the fifties era is what I'm thinking about.