I believe it would go in tight if not for the 4" barrel. The distance
between front of trigger guard and welt, and the security strap both
tell me 1/2" wouldn't do it. 1" probably. But no such thing as a 3"
N-Frame with fixed sight?

The 2" M&P is still the best fit, as marked, for standard issue
revolvers, but enough length to accomodate 3". Maybe a special
order? Maybe extra length to make room for the carver? Maybe
just a mistake?

Hey! Holding it under a much better light, for the first time, I just
noticed a 3" (very faint) stamped right below the 542. You can
almost see it in my 2nd from left photo above. Special order!

__________________
In Omnia Paratus