Measure the worn out one and the opening you need it to fit. Here are some 79 5.0 grommets but at least it has dimensions and you should be able to see if they are the same. Looks like two different grommets.
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(Fyi, used 1980 for part search since the 1979 parts data was screwy.)
Thanks, I've ordered some, but the shipping will take more than a month to ship to Europe.
Meanwhile we took measurements and are going to 3D-print some out of heat-resistant rubber. I'll tell you how it goes and which one was the right one.
Update 1:
We've designed and 3D-printed the missing part out of heat (and oil) resistant thermoplastic polyurethane (TPU). Fits snugly and should work until the proper part arrives.