Originally Posted by 97 SL320

Based on my research, it is for a 99 / 02 if it was sold to you as fitting a R129. That makes for 3 different mirrors 90 to 95 ish ( deep body line , rectangular mirror ) , 96 to 98 ( shallow body line , rectangular mirror ) and 99 to 02 ( smooth body , oval mirror ).

Have a look on car-part.com for some pics.

In the area of the yellow marker, look at the squares to determine when the housing was made and a part number that starts off with 129.