This kit allows you to synchronize your SU carburetors by comparing the piston lift in the vacuum chambers with a pair of dial indicators and adjusting the linkages accordingly.  The kit includes 2 brass collars with thumbscrews, a pair of dial indicators and three pairs of different length indicator extensions.
This kit is appropriate for all cars with H*, HS* and HD* series carburetors.  This includes all SU carbureted cars except for Silver Shadow II's which run HiF7's.
There is a separate kit available for HiF7's.
At the ten-minute mark of this YouTube video, the operation of the kit is demonstrated.