Reading a B&M Hydro tag

I have two B&M Hydros, one tag reads; P55 H7 UC3973, the other reads O53 D5 C 519?? . The O53 is a flat pan the P55 slant pan , so I an guessing that number is Olds (O) and Pont. (P) then maybe "tail housings" H7 H5 ?..........but whats the UC and C numbers all about ? thanks

 

I believe the UC is "unblown compe***ion" , there was an older post on that somewhere on hamb.

 

www.fatsco.net rebuild them and might know?