I don't think that Winchester ever made a 300 Wby, but I could be wrong.

Wsm's are in a short action and the Win. Mag is in a long action.

The best thing though is to call winchester and check the serial # against there records.

If its been rebarreled he may need a gunsmith to make a mold of the chamber to find out for sure.

Don't forget about the 300 H&H.