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None of the NAMC’s “Catalog of all Series Matchbox Models” (second edition), the AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983), Stannard, Houghton, or Leake ("Concise Catalogue of 75 Series Matchbox Toys by Lesney" (4th edition, reprinted 2006)) lists any variation not already catalogued by Nick.
U.K. Matchbox examined the model in Vol. 3 no. 6 (Aug. '79) p107 but did not record any variations not already identified by Nick.

ADDENDUM: Lesney Link no. 3 (Feb. ’86) p68 carries a report of an 18b fitted with the grey tracks normally associated with the 49a Half-Track. No second example appears to have been found and it would obviously be easy to simply swap the tracks over.

Idris wrote: ADDENDUM: Lesney Link no. 3 (Feb. ’86) p68 carries a report of an 18b fitted with the grey tracks normally associated with the 49a Half-Track. No second example appears to have been found and it would obviously be easy to simply swap the tracks over.

Not without cutting the tracks or taking the blade off, which means removing the front axle. One to look for!