Training grenade, couple of companies make them in the US, between $20-$30 new, but subject to itar apparently so not easy to ship over.
Regards sticking it on the kit, fuck that if it's that heavy, plenty of lightweight copies available online that'd look as good without the weight penalty.
I'd keep it as a paperweight or coffee table talking point lol, tell visitors your grandad bought it back from Normandy, go to hand it to them & then fumble it for a reaction 😂