After cleaning.
The wheel & hub were very dusty and oily.
The hub is part # C523/1S
It appears to be a non-Momo brand (don't know who made it) hub adaptor for BMW, pre-airbag.
Supposedly it fits:
BMW E30 3 Series,
BMW E28 5 Series (From 1986)
BMW E34 5 Series (From 1988)
BMW E24 6 Series (From 1987)
BMW E38 7 Series (From 1987)
BMW Z1 (Without airbag)
Here's what the Momo version of the hub adaptor looks like:
I happen to have this horn button.
It doesn't quite fit (won't sit down flush).
Will find another one to fit.
Will use a suede brush to clean up the leather, later.
After cleaning the suede leather (using a suede brush).