This kit allows you to synchronize your SU carburetors by comparing the piston lift in the vacuum chambers with dial indicators and adjusting the airflow accordingly.  The kit includes 2 aluminum collars with thumbscrews, a pair of dial indicators and a pair of indicator extensions.
This kit is appropriate for '77 and newer Shadow II's, Silver Wraiths, carbureted Spirit/Spurs and other cars with SU HIF7's.
At the ten-minute mark of this YouTube video, the operation of the kit is demonstrated.