Ref JulianS' query on sliprings . . . it's true there were some modest variations in the length of the brass strips on different rings. Quite why I don't know for sure, but the single critical thing is that an HT brush - one only! - must be fully on the brass at the firing point for that cylinder.
With manual advance and retard, the brush will be on a different spot on the ring according to the position of the camring (the position of which relative to the armature controls the exact moment at which the contact breaker opens). That variable may explain the use of slightly longer segments on some models, especially if using more advance and retard than is typically found on a standard K2F (which is around 18-20° on the mag via a notch c. 11mm wide limiting the camring's movement).
As long as one HT brush is in full contact with the brass segment at all positions of a manual camring (set correctly in its housing) when the points open, and as long as there is no possibility of the spark's trying to hop across to the other brush at the same time (owing to an overlong brass segment connecting both brushes, or nearly doing so), all will be well.
The sliprings made by the best UK supplier for both clockwise and anti-clockwise rotation twins use a strip of about 90° or a bit more maybe of the circumference, handed accordingly. This is adequate for correct operation of any manual K series mag I have seen, and also works perfectly OK on 50° V twins with an ATD. Haven't had the opportunity to see if there's enough brass on them to work in all circs on a pre-ATD V twin, like an HRD, but suspect there is probably just enough provided there isn't too much movement available on the camring or that the movement is slightly restricted to avoid under- or over-shooting the brass at the firing points.
The 35Kv marking refers to the dielectric strength of the material - ie its resistance to leakage at high voltage.
While it is common to see 25° or even 30° sometimes of advance/retard on single cylinder mags (which in most cases have a continuous brass strip through 360°), there isn't any real need for it I don't think except possibly for trials riders, and no Lucas twin I have seen has that much.
So the modern replacements available can be reckoned to work on all the models we're likely to encounter, with the possible caveat concerning manual advance V twin engines, and their resistance to leakage is as good as we'll ever need. Testing them at elevated temperatures linked in series with ignition coils operating at considerably higher voltage than your average magneto has revealed no problems that I am aware of, and I don't know of a manufacturer whose slipring products reflect the variations shown in the pix. Which isn't to say there isn't such a guru out there, natch, but I don't think there is any need for the different length segments for normal purposes. In cases where it is important to be able to position a brass segment in a specific way, there is a manufacturer in the Netherlands who supplies very handy kit-form sliprings which can be assembled with the brass strip wherever you like. The ones I have used are for Bosch instruments, and cater for parallel twin and V twin engines.
