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The NAMC’s “Catalog of all Series Matchbox Models” (second edition) notes that a darker metallic blue can be found on Nick's variation code 2 (i.e. the model with towguide).
The AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983) contains no variations not already listed by Nick, and neither does Stannard.
Houghton has no new variations either, and comments that ""colour shades vary from light metallic blue to dark metallic blue" for both the version with and the version without the towguide.
Leake ("Concise Catalogue of 75 Series Matchbox Toys by Lesney" (4th edition, reprinted 2006)) notes shades of the metallic blue exist, but does not link the shade to the presence or absence of the towguide.
U.K. Matchbox did not examine the model.

There is a small unlisted windows variation:
a) without ribs
b) with ribs

The windows rivet could be:
a) riveted
b) unriveted
c) chisel rivet
Antonin