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The NAMC’s “Catalog of all Series Matchbox Models” (first edition) has a variation at 57-1b not covered by Nick. It is listed as having SPW (presumably 9 x 20), crimped axles and silver trim. Without further details, it is difficult to see where it would fit in Nick's variation table.
The AIM “1-75 Series Regular Wheels” catalogue (second printing, 1983), Stannard and Houghton do not list any variations not already covered by Nick.
U.K. Matchbox did not cover the model.

Bob kept this model variation with SPW listed in the second edition of his NAMC catalog Hugh, but still did not have the model in his own very detailed collection. Chances are, this SPW model is held by a secretive collector and it may sometime come to market. Until then, without confirmation and a second one found, the SPW model remains a mystery to us that Nick may want to mention as seeking info on it's existence in his listings. (Or Not.....) Antonin?? kwakers