This kit allows you to synchronize the carburetors by comparing the height of the piston lift in the vacuum chambers simultaneously and adjusting the airflow to match. The kit includes 3 brass collars with thumbscrews, a trio of dial indicators and indicator extensions to reach to the bottom of the vacuum chamber pistons.
This kit is appropriate for SU HD8 carbureted Jaguar E-Types with triple HD8's - or anything else with triple HD8's. 
At the ten-minute mark of this YouTube video, the operation of the kit is demonstrated.