Well, the tach cable arrived. I asked the vendor, Bench Mark Works, if it was a genuine BMW product and was told that it was. It appears to be a very well made item. The product code was 62121357732 cable tach R60/6-R100RS.
The cost was $30.50 including shipping compared to the $72 quoted at the dealership.
Thanks once more to the gentleman who referred me to this source.