The NAMC’s “Catalog of all Series Matchbox Models” (second edition) notes that existence of a tan model (first body casting) as variation 38-1A. This model is also included in the AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983) as variation 38-1aa which is referenced in a footnote as being a possible preproduction. It is possible that both these models are, in fact, the first issue in grey/brown. Certainly NAMC do not catalogue that colour separately, and neither does AIM. However, Dick (Kwakers) did make a passing reference to an example of this model in tan in one of his posts (which I now can't find).
The AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983) also notes a dark grey model as variation 38-1a (type 1 body, MW, c/a). In addition, early models are listed with orange decals whilst later ones are shown having yellow, with the change occurring just before the move to rounded axles A crossover model is noted as variation 38-1g (Nick's variation code 11 or 12) which has r/a and orange decals. Stannard confirms the two decal colours, but calls the earlier colour orange/yellow. AIM notes the existence of light and dark GPW on this model (something which was missed by NAMC). Although all four body types are illustrated, only three of them are referenced in the variation table!
Stannard does not list any new variations, but Houghton has variation 38a-10 as a silver model with type 4 body, but with MW on domed r/a axles! This looks highly implausible to me.
U.K. Matchbox did not examine the model in any depth, merely commenting on the addition of decal guides in vol. 1, no. 12 (Feb. '79) p 194.