Help With History

I have in my possession and have seen many of these cases. I have never seen a loaded round. Can anyone identify and give me a history. Beside, on the left, is a 12 ga. for comparison.
Rim Dia 1.700"
Base Dia 1.565"
Neck Dia 1.557"
OAL 3.815"

Hello 30-30

I have a similar cartridge and was going to post it on the forum but never got around to it. Mine is like yours but instead of the R-P headstamp it has a “W”. When mine was given to me it came with the attached paper to authenticate it. I always assumed it was a fake. The paper appeared to be a copy of a copy of a copy. The reference to “0 Guage” appeared to me to be written over an area the had been subjected to"white out". The head stamp appeared to be crude and done by hand. I figured it was actually some type of 37mm flare case that some one doctored up but I really don’t know for sure.