The Shimano date code KB on the brake lever in your first photo gives Feb 1986 as the manufacturing
date of the lever , so your bike was made some time after that date . In the same photo the thumbshifter
looks to be an EM series SL-MT 50 with a chromed steel base , an aluminium shifter and a plastic top cover
This thumbshifter is the same as those fitted to my 501 model of the same date with a lower spec drivetrain
but otherwise similar . If the LH shifter is the same , it may be that the higher spec XT shifters were
not available at the time of manufacture and a lower spec was substituted ?