The BW ODs were in a separate case bolted to the transmission main case. The main shaft leaving the trans in high gear (usually 3rd) was the standard 1 to 1. The special longer main shaft drove a set of planetary gears, 3 in the R10 and 4 in the R11. The overdrive ratio was attained by the increased ratio through the planetary unit. The output shaft of the OD turned about 30% faster than the input from the transmission. If someone wanted to play with these they could become a stand alone box with a modified front plate. There was a GM OD 4 speed where 4th was the OD sort of like the T-5 except the OD was in the main box not in a special housing on the back. Attention must be payed to lubrication of the old BW ODs. Some modern gear lubes will eat the bronze parts. GL-1 works.
