Hey all I kept this master cylinder from my '72 Supreme when I swapped it out for a repop, and was wondering if it's original. The only markings/numbers I could find are pictured below.

 

The seven digit number starting with 54 tells me it is an original Moraine master cylinder casting .
That particular casting may have been used in several different master cylinders .
So it is not specific to any particular year or model .

 

5470409. Casting number. Used from 70-72 at least) The 1 1/8 for bore size, then the cast number itself (looks like mold # D77 used) Over on the end looks like the date code.... 133 or 135 it appears? Could be wrong on that. Hard to see. Which, if original, means it MAY have been sometime in middle of May 71 or 72 when the part was cast? No years were cast into these, unfortunately. There's also a half moon cut on them somewhere in the front topside that has a 2 letter stamp code that everyone goes crazy for. I know the 1970 guys think that EB is the shizznit. Normally, over the parts counter replacements do not have the 2 letter codes.

5472379 is the part number in the parts catalog for 71/72 w/disc
5472380 is the part number in the parts catalog for 71/72 w/drum

 

To expand on the correct info others have posted.
Original 1972 power disc brake masters have a two letter code on the top front on the master.
The code is "BE".
The original ones have bleeders, but as time passed they deleted the bleeders and the two letter code also.
They did however continue to use the same casting part number of 5470409

 

Would that 2 letter code be just below the cap?

 

It is basically.
you will see a machined pad that the two letter code is stamped in.