Not true (re legality). Some (but not all) MB's in the USA prior to 1985 had rear fogs -- it was dropped as a USA feature then, and wasn't reintroduced in any models until 1991 (when it became standard in all models here in the USA). However, in 1986, all USA Volvos and some other luxury makes (e.g., Jaguars) started included them.
MB also only uses a rear fog on the left side -- most other makes use a pair of them.
If you have that bulb in that socket, you've probably got the feature (because you have the electrical contacts necessary to keep the bulb securely in place -- cars without also lack the metal contacts in the socket).
To see if you have the feature, turn your lights on and pull the switch out to the *SECOND* detent (the first detent only operates the front fog lights; the second turns on both front and also rears). When you turn on your rear fogs, if they work the little yellow window in the middle of the light switch should also light up -- it's a required indicator to remind you that your rear fogs are on.
Let us know what happened.