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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, or Leake ("Concise Catalogue of 75 Series Matchbox Toys by Lesney" (4th edition, reprinted 2006)) lists any variation not already catalogued by Nick.
Houghton notes that both thin-boom variations (Nick's variation codes 1 and 2) also come without silver trim (Houghton's variation codes 13-d2 and 13-d4).
U.K. Matchbox did not examine the model.

It should be noted that Nick has expressed reservations regarding the authenticity of the yellow-hook model and, in this context, it is therefore interesting to note that neither of the contemporary catalogues (i.e. AIM and NAMC) lists this variation.

The reverse colour model is pretty rare and very expensive and therefore there is a lot of fakes. I think that the best aid to distinguish a genuine model from the fake is the comparison of the form of the ejector ring (and the thin boom of the jib of course). The very early models have a concave ejector ring. The later models have a flat or convex ejector ring. A problem is to distinguish a later reverse colour "fake" models assembled by workers at the Lesney factory from a real fake models.
Antonin

The genuine model with the concave ejector ring and a smaller gap

The fake

a different ejector rings

Great reference and comparison photos, thanks for posting.

Perhaps the most interesting model in Nicks listing is his variation code 2: normal colours but with thin booms.
My understanding is that this is presently only known from the box art, which is clearly redrawn rather than simply a recolouring of the reverse wrecker illustration. Since we know that the artwork was prepared from models sent to the artist for that purpose (hence the regular occurrence of pre-production colours on first run boxes), I think we can conclude that this model definitely once existed. Furthermore, the fact that Mick (SMS88) found a 13d with a repainted baseplate makes it all the more likely that first-type cab and boom castings were similarly oversprayed and used.
Has anyone ever seen one? Would anyone like to put their hand up to actually owning one?

would this model be as popular as it is if the reverse color model didnt exist?