Beginning in 1981 the 3.8L M116 (SL, SE, SEC, SEL) was imported by MBNA. In 81-83 the 5.0L M117 (SL, SEC, SEL) was brought to the US in substantial numbers in gray market vehicles. In 84-85 MBNA got wise and the 500SEL became a US spec import.
The OP's car is a 1984 500SEL.