A thread just went up with a 6" Model 15-5
and it has the wide rib barrel.

In general a revision change denotes a new part change, the 15-4 and older had the old narrow rib barrel and part of fitting that barrel was notching the frames square edges at the front,
This was not needed with the wide rib barrel.
The older Model 15 frames cannot use the wide rib barrel because it would not cosmetically blend there.

https://smith-wessonforum.com/s-w-re...#post141980663