I've attempted to upload and then attach a picture to this post. If not here then it's in the tech gallery titled 'tach for 122'. It looks like the post is 'outside' the body of the housing and some disassembly is necessary to properly install the post through the slot. [The guy I bought this from said hegot this when he had worked for a Volvo dealer - and that this was a customer's which was sent out for to be repaired and never picked up. It seems a little strange that someone repairing a tach would have reassembled it improperly and I haven't even tried to see how accurate it is - probably should do that before I spend any more time trying to make parts to mount it]
In any event it looks like "classic volvo shop's" photos are almost actual size and so I can measure from those to get dimensions to fabricate a replacement foot and wedge. Any ideas on what material would look good and not be subject to sun deterioration? I'm thinking black to match pad and tach.

It has a number on label on the bottom which reads either 810 or 018, depending....