You are right, sometime ago I found and linked the 1990 Lightweight catalogue here. Prior to this my knowledge was based on a friends bike which was running M550 LX, which as he'd got it used was probably upgraded. I don't think M550 was actually used in 1990 on the lightweights as the Blueridge ran 400LX (the was another below it with 300LX in the lightweight range in grey with a fluro green fork but it never made the catalogue), the Thunder road was XCD or E, the Moonshine DX and the White Lighning XC Pro. The LX and XT bikes must have been the new Dynatechs that year.