Probably the camera angle but it doesn't look like the tape is completely up against the cylinder, like the actual measurement might be a bit over 4". Which, given the none S&W front sight would be another indication it was cut down.

H Richard, do you think the factory would have put a rifle ramp on the barrel after cutting it from 6" to ~4"? Doesn't seem likely. More likely is that a gunsmith some where cut the barrel and put that sight on at the then owners request. In other words, he had it done locally.

All speculation on my part of course. It's still a neat ol' Heavy Duty.

Dave

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RSVN '69-'71
PCSD (Ret)