There can be no doubt that a Myford ML7 fitted with a Tri-leva speed-change unit is a highly-effective small lathe, and one in good condition is well worth seeking out. The Tri-leva was a cleverly-designed, exceptionally well-engineered unit designed to be mounted on the headstock of an ML7 (but not Super 7) to perform both as a clutch and also to allow instant changes of spindle speed made; it could be supplied ready-fitted to a new machine, or purchased as an aftermarket accessory.
The device consisted of a lower housing, in die-cast aluminium, that fastened into place under the headstock spindle by the use of three modified headstock-mounting bolts, each of which was threaded at both ends. The lower housing served both a mounting shelf for the upper part of the unit and as a guide for three permanently-mounted drive belts that formed the heart of the conversion. An upper housing held a similar belt guide together with three spring-loaded roller selectors, each of which could be pressed against the belt that ran beneath them. As one lever was pressed down an interlock automatically disengaged the one previously engaged.  The Tri-leva performed best when driven by a two-speed motor, this giving six high speeds (and six low in backgear) that could be selected instantly by a combination of mechanical and electrical control  Many professional users, especially those intending to use the lathe for production work in the guise of the C7 Capstan Model, chose this option and found the lathe to be perfectly capable of sustained hard work (on small parts) over many years. Unfortunately, the entire set up was relatively expensive - in 1962 a Tri-leva unit, two-speed motor and its associated electrical-control gear added 35% to the £70 : 15 :  0d cost of  a standard machine.
Although the solid-mount, 2-speed motor was much heavier than the standard unit, the same mounting plate was used. As might be expected, this objected to the extra strain and has been known to crack. However, this can be prevented by adding a simple support bracket between the plate and top of the stand.