I don't know all the details of what has been done to your car.
IF the dealer did a service at 7200 as you state, they probably at that point reset the FSS system. A "B" service always follows an "A" service. When you reset the FSS when it has one "wrench" (A service) showing, it will automatically show a "double wrench" (B service indicator). When you reset it at a "B" service, it'll show a single wrench symbol again. It can, at a later date, show a double wrench symbol, if for example you go over I believe 14,000 miles since the last service.
On the filter, yes, the charcoal filter on the 203 C320 is roughly where the DUST filter was on all 202 chassis. On all model 203 C class cars, the DUST filter is located above the battery.
On the work that was done to repair the oil level sensor, I believe it was simply being overfilled. MB now advises us that the correct amount of oil to put in after draining the oil and replacing the oil filter is 8 quarts, previous recommendation on that chassis was 8.5 quarts. ONLY on the 203 C class cars, other chassis utilizing this engine type is still 8.5 quarts.
Oh, but wait, you didn't change the oil prior to the sensor being replaced, or did you? Hmmmmm.
ps Just to clarify, the technician, when performing the FSS maintenance, should determine if the car needs an A or B service. On your car, it's as easy as seeing if one wrench or the double wrench is being displayed. Whatever the FSS is calling for, that's the service which should be performed.