The NAMC’s “Catalog of all Series Matchbox Models” (second edition) indicates the existence of two types of GPW (both dark grey): KGPW (which is presumably the standard 9 x 20), and GPW (which must have a finer tread). The latter is r/a and must therefore be the equivalent of Nick's variation code 8 (d/h, r/a, gloss black baseplate, with dashboard brace and two ejector rings).
None of the AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983), Stannard or Houghton contains any variations not already listed by Nick.
U.K. Matchbox did not examine the model.
ADDENDUM: Lesney Link no. 7 (Mar. ’87) p215 has a story about an example being found in brick red at a flea market in Hackney. Given the location, it was assumed to be a pre-production.