The UN72 BB was unique in that it had 2 loose cups. The UN73 is same as all the others, the RH side is a fixed cup.
The idea behind the UN72 BB is that you get a set of Phil Wood cups to go with the UN72 BB, discarding the original UN72 cups. The Phil Wood cups can be ordered in 26 threads per inch (26TPI).
So the UN73 won't work unless you get it re-threaded. Then I'm also unsure about what the imact on shell width is. I had mine rethreaded in the end as well as narrowing the shell to the standard 68mm to give me maximum flexibility.