It looks like its 1" but cant be sure. Its got 990s so its not a very early one*.
If it is 1", its probably a mk2 dirtbike, any year from 1992/3 to 1996. Other clues would help age it, like frame number, dropout style (one think one, or 2 thin ones stuck together).
*mk2s came with and without 990s, I dont think mk1s ever came with mounts.
Hope that helps