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Nick lists only one variation code for this model.
The NAMC’s “Catalog of all Series Matchbox Models” (second edition) does not contain any further variations, and neither does the AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983). The same is true of Stannard and Houghton. Even Leake ("Concise Catalogue of 75 Series Matchbox Toys by Lesney" (4th edition, reprinted 2006)) could not find anything to note.
Unsurprisingly, given the uninteresting subject matter, U.K. Matchbox did not examine the model.

26c GMC Tipper Truck is pretty boring model with no variation. Perhaps only two variations with the mold number "1" and "2" are knowm.
Antonin

But there is one unlisted casting variation on the engine block:
a) short side engine braces

b) long side engine braces without upper protrusion

c) long side engine braces with upper protrusion

d) late casting variation with long side engine braces could have a ribs

There are two casting variations on the Pre-Pro model

The tipper bed without strengthening rib

The inner rear side of the cab is without any strengthening. I suppose that there could be some standard red/grey models with this Pre-Pro casting variations. Unfortunatelly I have no one in my collection
Antonin

That blue cab picture is a bit out of Focus Antonin, but I do understand the lack of upper inner cab supports you are showing us. I was offered this same blue Cabbed Pre-pro model just 3 years ago now out of a good friend's collection, and I sure wish I had looked it over to find these two major casting variations on it. I am now sure that his model came out of Joe Stone's collection back in the early 70s and was in fact genuine. These paint trial/pre-pro models are of such poor paint quality they can be passed up for fear of buying a fake as I did on this one. Thanks for a lesson learned to now inspect and compare well before passing on any pre-pro "Special" ever again. What a prize Antonin! I guess we will all be picking up this ugly "Common" at Toy Shows now to find any stock colored models with these pre-pro early casting Cab and/or Beds that may have gone out during early production of this Truck. Thanks for your casting variations also Antonin, my old eyes will now be wide open to find all of them. kwakers
So much for no one finding any casting variations on these Hugh.....LOL