UPDATE: Here is a brochure and some directions/schematics for the rare Sierra Dana 20 Overdrive.

The Sierra Overdrive was designed to mount to a Dana 20.

In November of 1980, Pickup, Van & 4WD Magazine published this article on overdrives, which includes several I hadn’t heard about. This series of article images comes from Christian Harrison (and thanks to Marty Tilford for spotting it). The scans aren’t perfect, but are mostly readable. You can find a similar article at the Four Wheeler Network.

Particularly interesting to me is the Dana 20-mounted Sierra Overdrive. Willie Worthy of Four Wheeler was involved in the development of it, but noted that a lack of capital doomed it.