Not sure what year your T1 is, but I use this site - VW Parts - Jim Ellis Volkswagen Parts Department - to search for VW part numbers. I typed wiper arm in the search bar after entering my vehicle info and it brought up a couple of different wiper diagrams. Maybe you can find it that way.
It varies from year to year but I buy A LOT of parts online and always manage to find what I need. i used that diagram since VW every online dealer uses the same database and I don't see that part anywhere at all whether separately or in conjunction with another part such as #11 which does come with the arm. I have a new wiper arm in my cart and will call that dealer Monday to see if they can determine if those pieces come with the arm.
just figured someone here may have run across it and had an actual PN. We shall see.