0 gauge doesn't make a whole lot of sense. Shotshell gauges are based on number of round lead balls to the pound. A 12 gauge has a bore diameter such that a lead round ball that just fits would weigh 1/12 of a pound. A 4 gauge would have a 1/4 pound round ball while a 1 gauge round ball would weigh in at one pound.
I suppose a 0 gauge might mean something bigger, but how much bigger? For if you continue with the formula, it would mean an infinitely heavy ball. It could possibly mean something like a 1-1/8 pound ball, or 1-1/4, or . . . . A 00 gauge would then go up in some increment, followed by a 000 gauge by that same increment.
What does is the inside diameter of the case measure? That would tell us what the increment over 1 pound is.
ETA: It is not for a punt gun. Those were outlawed around the time of the self contained cartridge became into being. Also, the case is aluminum which was not even thought of as a material while these big guns were legal.