Ok, there is a chance I may have solved some of this on my own, but would love confirmation.
I went digging around my box of rubber things I pulled off of the car when I started its reconstruction and found 8 of these - 6 in good shape, two unusable.
They are a Volkswagen part used in the heater tubes of Beetles and are variously called "chassis plugs" or "rubber valves", the latter of which suggests that they are meant to act as a check valve to allow any water/condensation which collects in the structural tubes to drain out while preventing any water or schmutz getting in.
The rubber is actually pretty thin and bendy, consistent with the idea that a bit of water weight on top of them and they would bend and let the water out.
The P/N is 111 801 177 which is in PET in the right place, quantity 8 where they are called "rubber valve".
I tried one in one of the holes, they fit very well, not likely to come out and makes me think these might be the ones.
....confirmation would be lovely.
D.