1985 SM 500 and SM 600 Known for the use of a 24" wheel in the rear and the 26" up front. Supposed to aid in climbing. in 1992 they removed the "S" and they became the "M" you speak of. Aluminum freame steel fork Suntour Roller on the 600 and Suntour crivetrain. My 500 has the odyssey Pit Bull Roller on the rear and a Dia Compe BR MC 70 in front. Suntour XCM RD & FD Suntour 11-32 with Shimano Biopace 28 38 48 up front. Suntour Xpress Accushift Trigger shifters and in need of the handlsbars, headset and Fork-1" x 8.25" threaded Steerer tube.