Must be custom maker - no air vent lines. I saw a bunch of moulds like this years ago - 1970's - maybe 1980's.
Buddy Brad had one, a 4 cavity ,575" mould, that you poured 2 cavities, let the sprue harden, then turned the mould over and poured those two cavities full.
Shiloh(or something like that), I think it was called. I do not think it was the same company who made the Sharps rifles.