F26 = F250 4 wheel drive truck. Y = 352 Cubic inch V8 2bbl engine. R = built in San Jose, CA. 817734 = built in February, 1966. If you post the information on your driver door warranty plate, we can give you additional information.
Hi welcome to FTE. I looked at the pics of your truck in your profile. Looks to be a solid survivor just a little rough around the edges. Great to see another factory 4wd hitting the roads or at the very least being restored.
Here's the rest of your warranty plate decode: 120 = your wheelbase in inches. M = Wimbledon White exterior color. F260 = 6800 pound GVW rating 3/4 ton 4x4 truck. B81 = Blue Crush Vinyl and Blue cody pattern woven plastic upholstery in a Custom Cab. A = NP435 4 speed manual transmission. B5 = Dana 60 limited slip rear axle with 4.56 rear gears. 6800 = max gross vehicle weight rating. 172 @ 4000 = your NET horsepower and the rpms where achieved. 711779 = a Domestic Special Order vehicle, ordered from the Los Angeles District Sales Office. Unfortunately, the records that could say exactly what your special order entails are no longer available. Frequently, it has to do with special paint, but apparently not in your case as the Wimbledon White code is given and, from your album pictures, it certainly is painted Wimbledon White. Hope this helps.