The weird ones are that you could order an F-250 with a Dual Rear Wheel axle in the mid 80's as well. This option makes zero sense to me from a GAWR and payload rating stand point, but it was an option.
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You will have to prove that one. Show us the door sticker that shows that truck being a f250, and it will also have the axle code and the GVWR on the sticker. If it has the f250 letter in the vin, and the GVWR of a dually truck, I then will be a believer.
I called someone else out a long time ago on a single cab f250 4x4 having a dana 50 frontend from the factory. They showed their door sticker and sure enough, it was a single cab f250 with a 4400 lb front axle rating all on the same sticker. Those were special order also.
P.S. I have heard some guys on here talking about a aftermarket company who took f250's and modified them with a dually setup. You may find that is what the truck in the above picture is.