A question about 37mm projectile

I collected artillery brass and training devices in the 60s and 70s. I opened up the boxes a few months ago with the thought of selling them nut eBay does not accept even clearly labeled INERT items. (I’ve been selling my aviation collections on eBay for eighteen or nineteen years). I gave any I was uncomfortable with to the Sheriff’s bomb squad but found one 37-MM projectile that I’m unsure of. It’s a solid unfired projectile with a round brass “plug” on the aft end with two drilled holes. Marked “STEEL SHELL M6kk PF&M CO 37MM”. I don’t see any facility to add a photo but I have a clear one if necessary.