First year JP Transformers (1985) were numbered without C or D for Cybertron or Destron. They started that in year 2, as it became obvious that the numbers were gonna get huge quickly, and it helped differentiate between the good guys and bad guys.
The seller has listed it incorrectly in the title, but the packaging is legit. Reflector was released in JP in stores as '21', but was only a mailaway item in western markets by Hasbro.
Probably for marketting purposes. Maybe it was cheaper, or they didn't see the gimmick as being worth including, as Devastator would appear as a clumsy looking 'block' next to the rest of the toys they were releasing at that time (Turbomasters and Predators).
Shortly after, the US released the Constructicons with the combiner feature, probably because Devastator more easily fit right in visually with the various other Gen1 redecos in Gen2.
I think visually, from a marketting perspective, the UK didn't want Devastator, just the Constructicons, but the US wanted Devastator because they blended in with all the other Gen1 moulds being used.
But that's just my guess, based on how Hasbro prioritise the interests of retailers, who don't care about the product as long as it looks good on a display for them to look over and pre-order.