I also think the frames are different. The 2WD has an X frame and the 4WD has a ladder frame. Right?
I agree with the others about the C or K in serial number.
Those 2wd X frames with the front torsion bar suspension ran from 1960 to 1962. They changed to coil springs up front and a ladder frame in 1963 and stayed with that basic frame through 1972.
I've seen quite a few c10's and c20's converted to 4wd by removing the coil spring suspension and bolting/welding on leaf springs. My first truck was a '63 that someone had swapped on the springs and axles from a late '50's napco after removing the '63 front and rear suspension.
My most recent acquisition was titled as a '63 k10, but had a 4-5-6 cab, 350 th350 np208 from a square body, and axles from a squarebody. The frame and springs turned out to be factory '63 k10 that was just continually updated and changed through the years with different axles, engines, transmissions, transfer cases, and major body components.
Sometimes it's hard to tell with just pictures, and you just have to crawl under the truck to see what's been done.
