Well that's yet another one...My cap is about 1.2" across the threaded portion. I think most the cast iron cylinders are about 1 1/2".....there is a fellow selling the 1 1/2" versions on ebay...he had a bunch of them...
I've got one in the spare parts bin that came with my Jeep. It doesn't look quite the same as yours, and the cap is very different. I'll see if I can get a picture for you. I could also check if the cap from the other one fits it. If so maybe you could buy that instead.
I called and talked with Carl. Yes it's an original brake master cylinder, and no, parts are not available. Although the rebuild kit is the same as the cast iron units, there are no caps available. I made a new gasket for the top...the cap is serviceable it just looks like heck, as someone used a pair of water pump pliers to loosen it at one time...from the damage it must have been pretty well stuck..