Thanks to all who posted. Some excellent docs on the general subject. Still looking for more info on the plastic version. The search for knowledge continues!
Timmymac,
I also got one of these (dark)blue plastic ones in my collection.
The guy who sold it to me claimed it was an Israeli practice grenade, substitute for the M62's they use... not sure as i neither have any documentation....
Hope one day some doc's show up...