If you can manage to find a copy of the Ricambi catalog, it has most of the distributors available for Alfas. They are the same as what Jim is posting...
The difference between an S 166A and S166B is enough that they shouldn't be called the similar numbers... Max advance on an A- 18 deg, B- 14. Adv at 1000 dis rpm(2000 engine)- A- ~9, B- ~7. Ok, perhaps not that much different, but different enough.
If we didn't just upgrade our computer system here at work, I would scan both pages and post them. Thanks to progress, we no longer have a working scanner...
Oh, the Marelli Plex is almost identical to the 166B.
If you had a distributor bench, some springs, and a good book on how to blueprint and tune distributor curves, you could easily do it youself.
But I would probably by the upgrade offered by Centerline or RML
http://www.rmlautomotive.com/
Eric