Not only on our own Fiat 850 there were problems with the wiper mechanism, also elsewhere in the Netherlands a number of small parts were missing when work on the wiper mechanism was started. We solved it by making a hybrid system of two mechanisms. And because not everyone has the parts books, I publish here the chapter Tergicristallo from the parts manual. This way we have a reference to search with.
I've compared the 1968 and 1971 parts books and the only difference is that there was some extra development done by Marelli in 1971 making 4 extra part numbers available. See the table below
Fingers crossed that someone still has something lying around.