Barry -- When Dad passed the Suburban on to me, he also gave me all his data on it: one 3-ring binder about 2" thick, another about 3" thick, and an expandable folder about 3" thick. He saved every brochure, every receipt, every quote, every drawing he did, etc., during the planning stage, purchase, conversion, and maintenance/repair for the last 49 years.
While I haven't looked at every page yet, I did find the original invoice for purchase of the truck from Sauder-Lygrisse GMC in Wichita. It shows that the Watson overdrive was installed at the dealer when the truck was new for $370.05. At the same time the 305E was swapped for the 351C for $300.54. Total price for truck with all options selected was $4,483.96. Truck was ordered on March 9, 1965, and delivered on May 28, 1965.
There is a "Watson Brownie" brochure in one of the 3-ring binders, but it has no date on it. It does show the Model 2021 has a .82 ratio. I Googled "H. S. Watson Company" to try to find more info, but didn't come up with much. This was interesting, though:
http://forums.aths.org/PrintTopic31310.aspx.