Maybe they took the real PIAT's away from Ford because they found the workers shooting each others thermos flasks all over the plant.
A rubber stamp would certainly work well on reasonably small tasks. The item with the C Broad Arrow mark certainly looks like a rubber stamp that took a bit of a bounce, and the GRENADE marking has a stamped look to it as well.
The C Broad Arrow stamps came in quite a number of sizes, and I think I have only ever seen black or white stamps for it. Often wondered what they used for the marking fluid: an 'ink', regular paint or a special paint. When done on canvas, it does not seem to cake like paint. It soaks into the surface of the fabric and I don't think I have ever seen one of these stamps where too much fluid was applied. It always seems to be exactly the right amount.
Cheers for now,
David