I was going to say the same as Daedalus. Also, it should be remembered that the different plants (USA, UK and Mexico) had different stocks of keyboard parts (something easily noticed when taking into account the "inconsistency" in the dates of "adoption" of the differing logos, from silver square to oval gray to oval blue).
Since "industrial" keyboards were not made in huge numbers, it stands to reason that the Greenock plant simply had a large batch of gray cases in stock, which took long enough to be depleted, allowing for an, indeed, industrial gray keyboard with a black square logo to have been manufactured "as late as" 1990. Another factor that points in the same direction is the actual part number of this keyboard: 1388072, indeed lower than the well-known 1391401, and similar to the part number 1388032 (the US English industrial gray keyboard with a black square logo).
For contrast, I have a couple of Spanish Model M keyboards, part number 1391506,
with SDL ports and all. HOWEVER... they were made in 19
96 in the USA plant (not in Mexico), which at the time had already become LexMark and was normally producing Spanish Model M keyboards with the part number 82G2384 (with a fixed cable). The plant probably received an order for a batch of, specifically, 1391506 keyboards and manufactured those according to that spec.
So, Akephal, there is nothing weird about your keyboard. You indeed had a lucky find, and you should cherish it. Ensure the keyboard is in full working order and enjoy it for the decades to come!
