Osram 7506L (21w) "Long Life" Bulb (made in Germany)(6 available) - NOT to be confused with the standard Osram 7506 bulbs. I purchased these years ago when Osram/Sylvania still made them in Germany and when the price was reasonable. I now see these selling for $15+ on Ebay and ECS is selling the standard 7506 for $5+ on their site.

This is the bulb to use in your vintage BMW brake and reverse lights (fits newer models too). Only 6 left in stock.