G.M.S 20mm-M21 round


I have had in my possession for some time now an ammunition round with these details stamped on the base, then several lot numbers stamped on the shaft and head off the round.
On the shaft it has
LOT 50229-4
5 42
It has an orange and yellow shaft
On the head it has LOT-2-50424-214 the whole round is approximately 60mm long and doesn’t look like a convention style round.
Not sure how long I’ve had it or where it’s even come from, let alone what it was used in.
Some advice would be much appreciated. 15388288073515161788021366384653|690x1419