Don.

The Gradua system was designed by Zenith engineer Freddie Barnes, and manufactured for Zenith by the Advance Motor Manufacturing Company in Northampton.

Essentially the system worked by using an adjustable front pulley, the diameter of which could be varied by moving the tapered cheek plates closer together or further apart. As the front pulley diameter varied then obviously the belt tension changed and this was compensated by simultaneously moving the rear wheel fore and aft in the axle slots, this was achieved by means of a linked worm drive system.

I read somewhere that the effective gear ratio at the back wheel was infinitely adjustable between around 8:1 and 3.5:1