https://d2doctor.co.uk/grommet.pdf
I'm fairly sure the left hand side grommets came from existing production for something else as they are used on many other models. They were already available in proper vulcanised rubber so didn't need to be hand-cranked like the right hand side.
That said, they appeared to have used this same rubbish cast material on the B3, B4, C4 and B5 at least, based on requests I've had for other models, so we can't even put it down to the limited production of the D2.