looks impressive beside of a 45-70 but I dare to say the real big ole 577 BPE 3" will make it small again

because of the headstamp I looked for them https://www.rccbrass.com/?s=500&post_type=product
but RCC brass don't have it in the list.

the other suspect rocky mountain cartridge have a 56 50 romano in the list, must be the same person. don't ask me about the difference to the 56 50 spencer. http://rockymountaincartridge.com/index_htm_files/RMC%20Product%20List.pdf