Looks a bit like a resin cast to me too.
The dark contrast of the area, and the matching shapes inside a sink knob have always suggested negative space to me too. First I've seen of a letter there, im glad you brought it up, its always worth challenging a little
I read through this thread again and I don't think we ever got there, when it comes to this prop. Resin and Rubber stunts were brought up, but only in the sense that they existed.
View attachment 1054084
In this photo from Scott Juarez I see:
1) 8-9 scratches in the grenade, showing resin beneath
2) A thick, solid transition between Grenade and Balance Pipe
3) Solid Windvane
Clamp, Knob appear to be real, and I can't tell about the booster. As to why do it in parts... maybe they wanted to use a real graflex clamp, and knew it had to be "put together" because of the windvane, Balance Pipe and Booster (too wide).
It could be that the cast produced such bad Grenade Frag that Chaim's story happened, filling in the grooves and changing the grip sections, inspiring the stunts. They even cast the grenade upside down by accident. Conjecture though.