Here is the thread that I started about exactly a year ago, and it appears that it was probably a rolling change in 1992 - some models, like the 400E's, 500E's, and at least the diesels got R134a.
300E's didn't get it until MY1993, or so it appears...
When did r134a first show up in W124's?
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".