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There should also be a VIN tag riveted in the upper right hand corner of the firewall (passenger side) in the engine compartment. If both are missing or unreadable, then you'd have to look for the VIN stamped into either the left or right frame rails ahead of the front suspension. You'll likely need a wire brush to get rid of the dirt and surface rust on the rails.
If the first number is an 8, then it's a 48. If it's a 9, then it could either be a 49 or a 50. Only the production sequence number (the last five digits of the VIN) can hint to its year. I believe that if it's above 20,000 or 25,000, then it's a 50.
You might also find the rating plate on the inside of the glovebox door.
Look for something like 98RC 275000. That number is close to the break point between 49s and 50s. Or, 97HC________
They produced approx 145000 48s (+/- 10K as V8s and 6s were different) and continued the serial string from there.
From what I can tell from the phote it's a F-1. Heres a comparison of the truck in the photo and a F-1 with a F-2 fender. The wheel opening is larger on the F-2 finder.
Also, the truck looks to have the smaller, 5 lug brake drake of a F-1.
As for year, it could be any of the three years, 1948-50. I have seen '50s with the crank hole. The VIN plate, if it's still there, should answer all of your questions.
Is this truck for sale? Just wonder, did the guy tell you "it ran when it was parked"? That's my favorite line.