Having stripped the tailstock and saddle for cleaning and it was then that I noticed various unusual features: the wheel controlling the rise and fall has an adjustable central disc (Fig. 1) engraved with degree graduations - but (originally) no cursor against which to read the setting
So, the way the scale is used for angular positioning of the cross-slide is that the handwheel is rotated until the cursor reads zero on the rim's angular scale, the Allen screw A is tightened to clamp the feedscrew; the clamping bolt B slackened and the column rotated against the angular scale to allow the column to be rotated to the required position; finally, bolt B tightened again. (Fig 2).
…..Thus locked, when the cross-slide assembly was rotated, the handwheel rotated with it and the supposed 'vernier scale' was, in fact an angular scale showing degrees of rotation (Fig 3). Obviously the setting needs to be read against a cursor so I made a replacement - slightly different from the original in being secured to the bottom bracket of the rise-and-fall system by an adapter fitted in the threaded hole for the oil nipple (Fig 1). To return the cross-slide to 'zero' it would seem that 'clocking it' would be the best bet.