Found the best Nardi thread over at Nardi Dilemma - Page 11 - Mercedes-Benz Forum

Looks like the 390 MM would be better than a 360, and I don't have to take the steering wheel off, the 15mm uses a 22 mm socket, the 21 mm uses a 27mm socket. Great info. So all I have to do is take off the "Remove plastic Mercedes center emblem with a plastic tool or a screwdriver; pry under edge" Measure the nut and order adapter.

Every older Mercedes I have had, like the 1969 300 SEL 6.3, has had a Nardi, just like the look.