[Lynn Eades]

Lionel, I have no specific knowledge ( don't even know what the truck looks like) however it is probable that it was not even a pressure system in 1934.
At this point in my post, I did a little search, and found this:

The modern pressure cap The unsung hero of the modern vehicle cooling system must surely be John Karmazin of Harrison Radiators Division of General Motors in America. His invention was the radiator pressure cap which was first used in Buick cars in 1939, and has long since been taken for granted.

So, My guess was about right, that pressure systems came into being at the start of WWII. With G.M. behind it, the common standardized caps were the A.C. Delco types that you will be familiar with. My carrier has a design of cap that was a Ford thing specific to carriers, and before that there were valves that held pressure, but dumped the excess. the valves were fitted to the overflow pipe (1938 ish). If you think about the other M.V.s found on this forum, many were fitted with a catch tank that siphoned the water back into the rad when things cooled down.
I think your cap would have been made by the Rad manufacturer or by the truck manufacturer. Try setting up a search for one on ebay.
Good luck.