The gun was originally the "Registered Magnum" offered in 3.5,4,5,6,6.5,8.75" bbls.
The war interrupted production, but it resumed in '48. The Highway Patrolman (M28) came out in the mid-50's with a 4 and 6" bbl. Somewhere in there they dropped the 6.5 and the 4 for the 27.
In 1957 they redesignated the revolver line with model numbers, which is when you get M27.
In 1979 they dropped the 3.5" and 5" from the M27 and offered it in 4" and 6" and 8.75".
Pinned barrels and recessed cylinders left the scene in 1981.
In 1986 it was dropped in nickel.
In 1991 they dropped the 4" and 8.75" barrels.
In 1994 it was dropped for the catalouge.
In 1998 it came back as a stainless 8 shot (M27-7).
I don't know what all the different variation numbers mean.
Ross