The original spare parts catalog printed in May 1968 says A 000 997 16 94 while the current online parts catalog says A 000 997 78 94 without any reference to engine number or build year.
The latter part number is available from Febi Bilstein, they claim it fits all M100.980 and M100.981 engines from 1964 and onwards. Seems promising but I wouldn't know until I had tried.
Regards from Ingvar in Sweden.
109.018.12 1968 Bronze Braun (M100 in good shape)
126.039.1A 1988 Dunkelblau (M117 so a bit slower)
+ A few other MBs used for daily transport