AC filter introduction
all BMW - except E30- produced 09/1991 or later are 100% equipped with the microfilter. All E31 are 100% equipped with 2 filter since their start of the production.
BMW with microfilters are supposed to have a red maintenance reminder decal in the glove compartment. However, on BMW between 9/91 - 5/92 many of the labels were inadvertantly left off. These BMW are - however -equppped with a microfilter - see SI 64 12 92 (3518)

Some vehicle produced prior to 9/91 may be equipped with a microfilter, and can be positively identified by the maintenance reminder decal in the glove box.
Copied from a Service Information circular.