47/48 had three-point cab mounting, 49+ went to four-point mounting. Fuel tank is at the frame rail 47/48, then moved in-cab on 49. No big deal as long as you want to put the fuel tank in the cab of the 51. There is extra bracing,etc where the fuel tank would go in the early cab, so you can't just pop one in easily. Slightly narrower seat. Vent windows on the newer model doors, but the doors interchange.
