Naming the parts on it would help narrow down the year. Even if they aren't all original certain ones are very telling. Photos would obviously help. For example, and original 1982 version came with Mafac cantilevers, Tommaselli brake levers w/rubber boots, and a TA crankset. I had an '83 Sport and my sister has an original '82. My '83 Sport came with a Sugino AT crank, Shimano BR-MC70 cantilevers, and Avocet Touring saddle, Suntour Mountech RD, Specialized hubs w/Araya rims and Suntour Power Thummshifters to name some components.

So I found this 1982 original. https://budgetbicyclectr.com/1982-sp...icycle-22.html
By the serial number starting in M2, the 2 possibly stands for 1982. YOurs being an M3 likewise could be an '83. The parts will tell you if it's the Sport or more expensive "regular" Stumpjumper version.