I'm no expert but my "42" has a 3" chamber with a modified choke and I know for sure that it is just a field grade. I didn't even know they ever made a 2 1/2" chamber so it sounds like you have some sort of rare bird.
An original vent rib 2 1/2" gun is very scarce. Schwing's book may have a chart indicating how many were made in various configurations. I assume your gun is serial numbered 1954 production or later, the only period where Winchester marketed vent rib Model 42s.