I got this trench art type ashtray with some other items recently . It measures 4.38" across the base & has two indents either side of the primer toward the outer edge of the rim . Normally I can guess what these were made from as they are clearly marked but this has either been turned or rubbed bare . There is no extra thin band above the rim like on an 18pr I compared it with but it's about the same size . Out of interest , can any of the cartridge specialists on the forum ID it with such scant info ? Many thanks.