Couple of points here Swarfy.
First, not wanting to disagree with Black Sheep 'cos anything's possible (as we are seeing!) but very few stationary engines with Lucas or BTH or any other mags have advance and retard. They're happy at constant speed, like lawnmowers, so don't really need it. A typical single cylinder camring Lucas will have a square housing, but no AR facility, and that carried through also to the face-cam mags designed for stationary motors. Might well have had c/sunk screws though. So I don't think that was designed for a stationary engine. I think it was one man's way of recreating a part that would serve on a square-ended mag with that hole spacing. I think I see that the same part # item as yours appears in one of the pix put up by kiwipom.
Second thing is that apart from perhaps when you want a bit of retard on a hill, under heavy load with a pillion, it's actually not a great plan to use a manual camring to retard the mag in normal road use. Because by doing that you are spoiling the internal timing by making the points open late - further after the maximum flux change (flip point) than is optimal.
The spark drops off quite significantly with every few degrees of retard from the ideal - even a shrunken points gap will reduce the intensity slightly due to the retard it causes by delaying points opening. So, while it can be useful to run both AR systems in tandem it's not a real substitute for arcing the mounting holes if you want quick adjustment without pulling the drive off.
The purpose of the ATD devices we love and hate was to avoid weakening the spark by interfering with the internal operation of the mag (by, instead, adjusting the relative position of the elements of the gear train).
The best illustration of this is with V twins, where there is always one cylinder being fired depressingly late from the mag's point of view. If you take an HRD with manual magneto, and imagine that it is running one cylinder 25° 'late' at the mag (half the 50° V angle), then add another load of retard with a manual camring which will move about 18-20° on the mag, or 36-40° on the crank . . . then it's small wonder they can be hard to start as the spark performance is horrible diminished on the #2 cylinder. Vincent therefore were early adopters of ATDs, to get the most out of the mag that they could. Some V twin cars used twin mags for this very reason, as they can be bloody hard to start with a starting handle. Indian went to coil and distributor ignition for this reason, even before they had decent dynamos available to keep ignition coils and lights running.
Even on a K2F, if a good one does what Lucas says it should and delivers near-1/4inch sparks at atmospheric pressure on three-point test gaps at 130rpm at full advance, it will need almost for sure 150+rpm when retarded. Doesn't sound a lot of difference - but when turned into kickstart speed it means you need to be able to spin the crank at 300+ rpm by footpower to get a spark if using a lot of retard.
The fact that ATDs can be horrible darn things (and cause far more than their fair share of hassle as is demonstrated by the number of threads on here) doesn't detract from the key point that they are 'a good thing' from the point of view of maintaining top-strength sparks. If given an option, I'd always prefer an ATD over manual for that reason - and also for another important one, which is this: a fixed camring does'nt fret in its housing; if you've got it so it fires the mag at the correct interval (180:180° for us), then it's more likely to stay there than is a manual one with its risk of slop in the housing, fretting of the plunger in its notch, etc. The number of mags I see where the firing interval is several degrees off at the mag when tested dynamically - double that at the engine don't forget - is very high, and the manual versions are more susceptible. Worst case, you'll see 5°+ error at the mag, 10 at the crank and with a high compression motor, that can only be very bad. You can split the difference in cases of modest error when setting the timing, if you can measure it accurately, but you can't make a silk purse out of a sow's ear however you try! Hence . . . people making and selling replica parts to help us get round some of the problems.
