Didn't the swingarm and linkage change for the 1991/92 model? 7/90 would still be a 1990 model, and this bike has the earlier swingarm. I'm not sure of any difference between 89 and 90 models?
All early WR's had lighting coil, wide ratio gearbox and 18" rear wheel. I think a larger tank came out in 92.
Cheers, Grahame