I thought this auxiliary transmission comparison was interesting. A 1969 article promoted Warn’s Auxiliary Transmission, an overdrive that sat between the transmission and transferase. By early 1970 Husky was promoting a similar sized heavy-duty overdrive that sat between the engine and transmission.
More pics of the Warn Auxiliary Transmission. This one was sold in Nevada in 2013, priced at only $450 (also included tranny & tc).

This one was priced at $700 and sold out of Maryland:


















