They started with just the WSSMs and WSMs but ultimately, changed them all. They had some real problems with chambers which were oval when the barrel was torgued into the receivers which had the 1"x16tpi interupted thread. Hance the change to the 1 1/16x28 TPI thread. It only made sense to make all receivers the same to ease manufacturing set-up and this is what they did. Eliminating the interupted thread was a good move IMO. GD
