The originals were a bright orange sealing rubber that appeared to be part of the domed washer surround and centre, compressing upon install. There were separate parts numbers for the top sealing washers and the lower standard domed washers without the seal. There was also a different sealing washer added between the bumper brackets and the trunk sheetmetal on the outside, according to parts diagrams. I haven't seen the remains but was likely the same rubbery foam type washers that sealed similar holes from between spoilers and trunk lid, likely crumbled and disintegrated upon removal.
Adding some sort of sealant is always a good extra, around the outer edge but also the inner edge of studs. Some snug fitting neoprene rubber washers that compress, fit tight on studs and slightly larger than the washers, are also a good method, and can be added to both sides of sheetmetal.
Here's a used set on ebay of the originals just for reference. I'm not sure of any similar items available new for resto use.
https://www.ebay.com.au/itm/124933302545?hash=item1d169ae911:g:QOUAAOSwoLphWwxS