Mark played this guitar occasionally during the Love Over Gold tour in 1982/83, and for the song “Two Young Lovers”.
Not much is known about the guitar, unfortunately. It appears to be highly modified since it’s a SG Standard model with 3-pickups, which is not something you’ll find on factory models. You can find them nowadays, but this is most likely, based on the body shape, a vintage 60s model.
That SG belongs to Hal Lindes, according to Hal it was the first humbucker loaded guitar that Mark used in Dire Straits!