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The NAMC’s “Catalog of all Series Matchbox Models” (second edition) highlights that both light and dark GPW are found with r/a. Both the AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983) and Houghton concur, and indicate the c/a variation to have dark GPW.
Stannard does not list any variations not already covered by Nick.
U.K. Matchbox examined the model in vol. 3 no. 9 (Nov. '79) p 168, and has a new variation identical to Nick's variation code 1 (i.e. full gold trim, f/h, c/a, 8.5 x 18 MW) but with the drum shaft extension. Rob Newson has an interesting theory to explain this. His musings read as follows: "Somewhat surprisingly for a model with a six-year production lifespan, only one minor casting change took place on this model and it is argumentative on whether this was an actual change or only a repair to an initial faulty mould! This particular repair, or casting change, was concerned with the length of the barrel stem at the front (closed) end of the barrel. Some early casting exist with this stem cut short - i.e. a gap exists between the end of this foreshortened drum/barrel axle and the large cylinder behind the cab. However, other early castings show a continuous stem reaching this cylinder, and all subsequent castings are of this second type, thus we believe this 'short stem' type to be in the nature of a fault in one mould that was corrected to conform with the other moulds and the laid-down design". We could argue wheter Lesney was using twin moulds when the 26a was launched but, even if they weren't, Rob's argument still holds in terms of a fault developing on the single production mould and, after an unknown length of time and production of an unknown number of 'faulty' castings, being corrected. If confirmed, the new variation would become Nick's new variation code 2.

Over here in the U.S., the early short stem barrel variation has always been nearly non-existent in collections. Anyone wishing to get one of these early much weaker no-stem castings usually had to source them "New" from English collectors because that early stem design caused them to become two-piece toys if they were played with at all over here. In looking closely at a short stem Mixer I have with a 'removable' barrel, the stem design inside the barrel was made flat just as it was through the whole production of this 26A. Our short stem Mixer was hit hard enough with a peening tool at the factory in an attempt to secure the barrel to the Body casting to actually slightly bend the top of the short stem mount. Even with that amount of force, the peen was not enough to begin to secure the barrel at all. I can see why the barrel mount would have to be re-designed and made much stronger to allow a much heftier peening hit in order to properly secure these early small barrels. Why this was not done as a result of pre-production testing amazes me, just as the continuation of the "Flat" end design of this 26A barrel rivet has all these years. If Lesney had changed to a hollow barrel rivet design as was then being used to secure most Lesney model's bases, (And was in fact later used to easily secure the 26B very brittle plastic barrels), the spinning design rivet tool would have quickly secured all the 26A barrels far more effectively and with much less force put against the Truck chassis while doing so.
On the question of a 'shortened' long stem barrel, I have never seen one of these as he describes them at all. When looking and comparing the long and the short barrel stem castings, any defective long stem casting could never be mistaken for that distinctive short stem small dome that makes the earliest casting so easy to spot. That small dome is perfectly centered, perfectly rounded, so seems just as Stannard classified it, the earliest (And of course weakest) initial barrel stem design. If a non connected 'long stem' is eventually found by us Hugh with your new information, I would say it will be a much later code with a defective or incomplete stem, not a totally unknown till now interim 'early' casting. Newson calls the short stem "A Fault", and he is certainly correct on that part of his description of that early casting. His theory of 2 separate molds used initially with 2 different types of stems is certainly a strong possibility, but the short stem was updated very early in the 26As production because it is a very uncommon model to find in one piece today. Cheers! kwakers