Generally MB codes are the same for any given chassis/year/market, worldwide. You cannot have a code that denotes dust filters for one car sold in one market, and another code for cars sold in another. A code is the same, worldwide. It is either present, or not present.
The list of codes in my MB PKW-Verkaufen-Taschenbuch, dated October 1990, shows the following:
Code 305 - not shown in book. Another reference shows from 1-November-1990, that this is indeed a Dust Filter code
Code 580 (Klimaanlage / air conditioning) - standard at no extra cost. Code must not be prescribed. List price DM 3,910— for non-500Es.
Code 581 (Klimatsierungsautomatik / automatic climate control) - available as an option. List price DM 790— for 500Es.
What this means is that 500Es could be delivered with non-AC by specific request at no cost; or A/C (Tempmatic) as standard at no cost; or with ACC at a 790-mark extra cost. No wonder why so many non-US and non-Japan cars were spec'd with Tempmatic, but not ACC. The US didn't get a choice, it was ACC or nothing.
The real code to hone in on here is the Code 305, which indeed does specify dust or cabin filter for the W124 and other chassis, after late 1990. I still do not believe that any 500E built before the spring of 1992, would have cabin filters (perhaps unless it was a special request, hence Code 305 being present on a 1991 or 1992 500E?). And all cars built after the MOPF2 changeover (i.e. generalizing a bit, facelift cars), would come standard equipped with a dust filter.
My best guess is that the filters were perhaps originally spec'd as an optional item (Code 305), and starting in the spring of 1992, began being phased in as a standard production item for non-US-market 500Es, and then later became standard with the transition to MOPF2 for non-US cars and the "model year 1994" changeover specifically for US-market cars.
So, the quest for early 500E models with Code 305 is really where this quest needs to center. What was the earliest car with Code 305? And when and how often/many subsequent cars with Code 305 were made before the MOPF2 / US 1994 changeover?