That's not a 5. It's a blurred and worn out 8. As in an 806.
While the "506" is not really all that unheard of, there's a reason it's unseen. Most commonly, it's people confusing the 560. We had it happen a lot on our old farm. Some hands called the 460 a 406, too. When the 560 was replaced, they'd sometimes call the 660 a "606" as well. We had both an 806 and a 1206, so it's not surprising. Same folks called the old J.I. Case model LA the "LA Case". They were there because they were strong, energetic, and needed to pay their bills and feed their own families. Not because they were smart.
that said, the 606 was a real tractor, and the bigger brother to the IH 504. Fun fact, the 504 and 606 might have been the same series, but they end in different numbers because that's the number of cylinders in the engine.
So, any guesses to what grandpa Jack did when he jury-rigged a 606 front chassis and engine onto a 504 rear end and front carriage?
He commissioned a custom side number plate, reading "506".
We later updated it to say "506-BMF" (Big Maple Farms) and added a custom nameplate as well, reading "Farmall" on one line and "Jack Huls Edition" below it. Lastly, we put the old Big Maple Farms logo on the front sides, above the numberplate.
All this started due to Jack reading a magazine article about "Frankentractors" and the dedication their creators have to authenticity.