I measured the metal hubs on the K4a Tractor and compared them to the K-11a Tractor and they appear to be the same.
Dia. .649 in. or 16.4846 mm.
Thk .335 in. or 8.47 mm
Only difference is that one is painted green and the other orange.
Couple of great pictures James. We now know where the wheels came from and I assume those Green ones are also metal, assuming again the K-4a is the earlier issue model.
A natural progression from metal to plastic is common among our toy and model makers, it makes collecting variations such fun...