This professional universal double indicator kit is specifically for shops that have a specialty in classic British cars or the discerning aficionado with multiple SU aspirated cars in their collection.  There are 2 1" travel indicators and 2 pairs of threaded collars included with this kit to allow for all dual carbureted cars.  The brass pair of collars is threaded to fit all H/HD/HS/HV series and the green pair fits HIF series carbs.  Three different length of extensions are included.

At the ten-minute mark of this YouTube video, the operation of the balancing kit is demonstrated.