Dave Ross
I did not see your original question about "factory optioned" narrow frame tractors, but basically the answer is dealer installed, at least on the early narrow frames. The "Price List and Terms Schedule" for the 1965 70/100 list the tractors, decks, blades, snow throwers and etc., and the location a dealer would order each one from, such as: Tractors, Louisville, KY; Spring Trip Blade, Rear Blades, and Peg Tooth Harrows, Canton, IL; Mower Decks, Memphis, TN, etc, etc., but it list Three Point Hitch, Rear Wheel Weights, and Creeper Drive as "Attachments - Field", meaning they were a dealer item. The hydraulic lift unit was listed as a "parts accessory" when it was made available and sold as a Hydraulic Unit Parts Accessory and Adaptor Package (Hyd Unit was P/N 386135R91, and the Adaptor Package was P/N 386155R91 for the 70/100, and 386154R91 for the Original).
The tractors were built as tractors, and thats all you got. Then you bought a deck from the dealer if you wanted to cut grass, or you bought what ever attachment you needed to do the job you wanted the tractor to do. The same way with accessory parts, you bought them from the dealer and they were installed by the dealer or by the owner.
Unlike ordering a vehicle that was then shipped to your dealer, IH built tractors by the 100's, both bigger tractors and Cub Cadets, and shipped them to regional distribution centers for distribution to dealers in that region/area that wanted, or had a sale for, that model. In Louisville and surrounding area, dealers would come to the Louisville Plant and pick up Cub Cadets by the truck load.
So basically, if the item was not listed as standard equipment on a given model, it was installed by a dealer, or purchased from a dealer for owner installation.