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The various catalogues have some interesting combinations which are missing from Nick's listing.

The NAMC’s “Catalog of all Series Matchbox Models” (first edition) has a silver-trimmed version as Nick's variation code 1, but with "large GPW" at the front (72-1-a). This is followed by a silver-trimmed version of Nick's variation code 1 (72a-1-b).
The AIM “1-75 Series Regular Wheels” catalogue (second printing, 1983) has three curious entries. At 72-1-a is what appears to be a silver-trimmed example of Nick's variation code 1, but with grey, treadless rear tyres. Variation 72-1-b is either Nick's variation code 3 or 4 but with black, treadless rear tyres, whilst variation 72-1-c is Nick's variation code 5 but, once again, with black, treadless, rear tyres. (I do wonder whether these treadless tyres might not have been o-rings fitted by a previous owner as replacements for lost originals.)
Stannard seems to be covered by Nick's cataloguing, but Houghton’s “Definitive Guide to Matchbox Toys” (2007) lists Nick's variation codes 1 and 2 as being with silver trim (72a-1 and 72a-b).
Oddly, U.K. Matchbox did not examine the model.
Last but not least, our very own Johnnijhuis has shown a (presumably silver-trimmed) model with 11 x 45 BPW front wheels and 13 mm yellow hubs on the rear with what appear to be 22 x 30 black tyres on the rear. The base rivet casting is unknown, but is probably the central ridge, placing the model in Nick's table between variation codes 4 and 5.

The treadless early tires refer to the early design that we know as standard. The outer circumference is very flat, not like a 'Real Tractor Tire' which has deep high treads running diagonally across it's width to bite into the earth for traction. Treadless refers to the early 22X30 rear tires, but is confusing to us because on trucks and vehicles the Lesney 'treads' we count are cast into the sidewalls of the tires, just like these Early tractor tires. That was a very poor choice of wording used in those early guides. kwakers

Thank you for the clarification, Dick!
That means that the AIM anomalies resolve to a silver-trimmed version of Nick's variation code 1.

I believe we have seen the marketing of these Tractors with yellow rear wheels and solid front tires. Unless anyone can verify these were EVER produced, and has what they consider a genuine one, perhaps a warning at Nick's site might be appropriate. As we continue on in time and these continue to be sold, investors and newer collectors will eventually be confident that this variation is legitimate. Perhaps it is time to study it further 123..... kwakers

The 72a Fordson Tractor page has been updated.