To my knowledge the end caps are not switchable between your original and the repop MC. I went through this same issue when I had to replace my '51 Chevy 3100 MC and my original end cap and line fitting was the same as yours pictured.
After trying 3 different Classic Parts '51 specific MC's (with all 3 failing internally out of the box) they replaced the non-working '51 style unit with a '52-'55.1 MC to which they added a brass adapter fitting to work with the newer sized output plug on the MC to connect with the original brake line fitting. Classic's pictures don't show the fitting or a good view of the output port, but Jim Carter's offered
1952-55.1 1/2 ton MC pictures the same "B238" casting number on the MC and the brass adapter fitting that he provides with his.
I have a spare '52 style MC that came with the adapater squirreled away somewhere that I got from fellow Bolter Phil "Phak1" some time ago. If I can dig it out, I can try and get the measurements/thread count from the fitting and post so you can try and locate one.
Alternately, you could reach out to Jim Carter and see if they can provide the adapter info.
