YMMV, etc...
I'm no Expert either but do know a bit about the yellow insert frt sights, as I have a pair of 29-2's with them. Yellow inserts were available from the Factory, and I have seen a few in the past 40 yrs. But they are surely rarer than red.
At least back in the 70's and 80's if Smith had the parts, they would retrofit about any bbl, cylinder etc between like frame szies...if it was possible.
I sent an early 2 1/2" M-66 RB in through Rays Sports Shop in NJ and in two weeks had back a 4" gun. I carried it off duty that winter, then sold it through Rays as a consignment gun...for a VERY healthy price.
The possibility of your gun being rebarreled by the Factory is probably quite high.
FN in MT