On a 91, there were no letter-designation services from Mercedes. They were still done on mileage intervals, such as 7500 miles, 15,000 miles, 30,000 miles, etc. The intervals were every 7500 miles. A 7500 service is basically an oil change, but the other services, the 15000, 30000, 45000, 60,000 etc are more involved. There should be a 7500 service (oil change) between these services, such as 22,500, 37,500, 52,500, etc.
They had NO letter designations back then, not until 1998 or so when FSS came out.
If you have a receipt for a service from a dealership that is referred to as a "C" service, then this is something the dealership dreamt up on their own, not authorized by Mercedes (not that it matters that much, as long as the work was performed that was due at that interval, which I have no way of knowing). Call the dealer and ask them what a "C" service is on that car.