Should I make a seatbelt alteration?

[steveinfl]

I have a 1969 911e which in nice driver (because I drive it) condition. No modifications. I like it original, even the radio. I did however buy a retractable seat belt which I have not installed. When I bought it, it was just supposed to bolt in but to bolt in, I have to drill a hole through the rear quarter vinyl to bolt into the hole already present from the factory under the wheel well.

I am confused as to why there is a factory hole for retractable seat belts which were not yet available in a 1969 911?

Also, to use this hole I need to chip off the factory undercaoating under the wheel well and pop out a plug which is in the hole. I don't know if I want to compromise the waterproofing integrity by poking a hole in the wheel well.

Has any one done this who could offer advise?

Thanks
Steve