You have a 91 SL300 with an AMG badge stuck on it. SL is the model, and AMG is a high performance tuning company (now owned by MB, but seperate from Mercedes-Benz in 1991). AMG makes high performance models, very expensive. In 1991 MB made the SL two ways, the 6 cylinder SL300 and the V8 SL500. They did not make a AMG version.
Some people stick AMG badges on their Mercedes thinking it makes it special.