I used to have a synchronizer that I think was called a Uni-syn the had a screw in cone over a plate you held to the face of an individual carb. There was a pivoting clear tube with graduation marks you could adjust to be vertical and a ball that would float in the tube by vaccum. You screw the cone in and out to get the ball floating mid tube and frig with the linkage of all the carbs till they are all sucking even.
The biggest problem with old SU's is the shafts and the housings where the shafts go through wear and leak air. The cheap and easy fix is counterbore enough to get a greased O-ring in. The more elaborate is machine the housing and put in bushings. Some rebuild specialists put in new shafts as well. You can adjust the richness of individual carbs by looking at the spark plugs and sliding the needles up or down in the constant velocity slides to suit.
There was also some kind of indicator kit that went into the dashpot holes to synchronize by the height of the slides but I've never used them and don't know how well that worked. I remember there were different taper needles you could swap in.

 

i cant find any XKE water rails Xp