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The NAMC’s “Catalog of all Series Matchbox Models” (first edition) distinguishes three distinct shades for the side flashes on this model. "Pink" is the standard colour, but "butterscotch" and "bright pink" are also identified as being issued as per Nick's variation code 2 (9 x 20 SPW, dark blue base, green tinted windows, type 1 trim) in addition to the standard colour.
The AIM “1-75 Series Regular Wheels” catalogue (second printing, 1983) does not contain any unlisted variations, and neither does either Houghton’s “Definitive Guide to Matchbox Toys” (2007) or Stannard.
U.K. Matchbox examined the model in Volume 4 (December on p159, with a follow-up in Feb on p206). There is an interesting comment about different wheel thicknesses with differences of up to a factor two having being noted. Frustratingly, no details are provided. Once again, side flash colours are distinguished, with the standard colour being described as "orange", but both "buff" and "pale orange" are found associated with Nick's variation code 2 in addition to the standard colour, and "buff" and "orange" are listed as occurring with Nick's variation code 6 (6 x 24 SPW, dark blue base, green tinted windows, type 2 trim).

It therefore looks very much to me as though the models with dark blue base are associated with non-standard side flash colours. (However, it must be noted that the side flashes are very prone to fading, so we would need to be very sure of ourselves before electing to catalogue such variations.)The UKM comment about wheel thicknesses is also intriguing - can anyone please provide additional information?

All the wheels on my 75As are mainly in the 2.4-2.6mm range of thickness, both SPW and BPW. We apparently never found any GPW in circulation or at the Toy Shows we attended. I do have a couple of wheels at 2.2 in SPW and also at 2.9 in BPW as the extremes in measurements, but they just seem to be oddballs that may have flashing or 'high spots-low spots' causing them to measure a bit different. I do not have any of the later fat BPW or GPW that are Very wide on any of my variations, so maybe others will find they measure much wider than mine above.
As for side colors, natural light versus indoors effects my definitions on those. I took them to natural light and they did change my perception of the colors a bit. My blue bases tend to have orange-pink side panels, but maybe a couple of different shades best described as bright orange and orange-pink. My two black bases with BPW are both very different. One has bright Pink side panels while the second has VERY light pink side panels that are not bright at all as NAMC Bob's guide describes his. Most all of our Thunderbirds were out of stores in the 60s, and had never been known to be sunfaded. Even that being taken into account, the very light pink one shows just a bit brighter pink on it's second side, indicating it may have been a display model exposed to sunlight many years ago, bleaching it a bit on it's 'lightest' side. The second side is still light pink, but not quite as pale as the first. kwakers