Yes, you'll have to show them you're registration because the boob behind the parts counter won't know anything W123 or OM616/617 related .
They'll also have to order it in, why I gave you the proper size and no, YOU SHOULD NOT JUST USE TWO OF ONE SIZE like 'everyone else does'
.
If you have not yet discovered The Mercedes Classic Center you need to, there's one Down East and another in Irvine, Ca. ~ I use them because :
The always have the correct part that not only fits, it works like it should .
Also, they'll mail it to you making it the single easiest way to get parts, said parts being well made and the RIGHT ONES, often un available anywhere else .