Was it this one (copied below in italics) kiwipom (posted under 'Frankenstein tacho' in the 'Frame' section)?
But before that . . .On the housing fasteners . . . cb housings come to suit mag bodies with their holes tapped in various combinations - 2 or 3 of, 4 of in some cases.
Manual camring housings need three - the top right hand one will be there for a K2F being used with the AR cable entering on the left looking from the points end. This is for slack cable advance on a mag driven ACW from the drive end. The end housing designed for Clockwise mags to have slack cable for advance (or for ACW ones to have TIGHT advance if anyone uses such), has a threaded hole top left 'cos the cable and plunger enter on the right in an opposite-handed end-housing casting.
Multiple versions of end housing are out there, for manual camrings as well as for fixed - as shown in some of the relevant pix. Some have deep ears, some have shallow, some are designed for alloy dome end covers for the points which use special studs, some for large screw-on jobs, some for clip-on bakelite covers with or without kill-wire take-off facility. The number of different part numbers is amazing really, for the housings and also for the complete magneto. Given that the insides of K2Fs are all pretty much of a muchness, the different numbers come down to ACW vs Clockwise, type of housing and points cover, fixed or manual, square housing or rectangular, side of entry of cable, etc etc (as well as drive end axle diameter - 15 or 18mm). On which point, I'd always prefer to use an 18mm one with an ATD, whatever was original, because the number of 15mm axles which end up working loose in the armature end cheek is pretty scary after 60-odd years.
It is always possible to drill and tap a 2BA hole or two where needed to do a conversion BUT it does have to be done spot-on: the effect of a few thou error can be either a no-fit or stressed fit of the end-housing register into the mag body, or, if there is a fit thanks to a bit of slop, an error in the centralisation of the camring, which has consequences for the firing interval between sparks and for the points gap on the 2 cylinders. Sounds pathetically trivial, but truly it isn't, as a tiny error can screw up the timing between cyls by several degrees in a heartbeat. I have seen errors during dynamic testing of over 5° in extreme cases - which is 10°+ on the crankshaft and can destroy a decent engine if you think one cylinder might be at say 34° BTDC, but the other is massively advanced at 40-something or horribly retarded at 20-something.
A further thing to think about with buying 2nd hand housings, is 'does it have the eccentric pin to act as a stop for camring movement, or is it an early one with axial screws spaced to hold a large notch in the camring in a fixed position?' Best always to have the eccentric pin on manual mags rather than faff about. And, on the camring, is the notch for the AR plunger on the side you want it on - ie the LEFT, at roughly 9 o'clock - which you'd want on an A series?
The mag can be converted OK, but there is a little bit to do.
Change of end-housing at the cb end as mentioned;
change of camring for one with notches in it to allow for movement and to fit the operating plunger (you could mod the ring that's there, but probably need to polish / relieve the outside of the ring to get it to be a sliding fit not a tight one);
extract and refit bearing outer race into the manual housing;
buy AR cable and gubbins;
maybe have to find different cb end cover depending on type of housing sourced. The only sort that makes remote kill/ stop button difficult is the end with the alloy dome fitting with metal button - other types used on twins have the facility for a wire to go to 'bars. Even one of them can be converted to use a brush and spring in an insulated sleeve, with a wire off.
This can all be done with the mag in situ on the bike. But if it's coming off for a general checkover, then all well and good and a complete new bearing could be fitted at the same time probably, with the armature out.
With the manual set-up on an A, you might need to change the front HT pick-up if it fouls the AR cable entry. The usual 45°-angled pick-up (if it's got one of them) isn't deeply-enough cranked for clearance - a vertical one will go on, just, and there are cranked ones available, though their quality isn't always too special.
Then you need your fixed gear - there were some neat alloy ones out there at one point (SRM I think), probably still are - and a non-Frankenstein nut for the tacho drive.
The most expensive thing there would be a replacement manual camring if you went for new, so I'd mod the old one if it's any good. Dremel or similar fine grinder and steady hands or bench fixture to enlarge the notch that limits movement, and then to make the V slot for the cable plunger. They do have to be in the right spots, or it's TROUBLE. Elongate the notch that's already there to about 10-11mm long to provide retard only - ie to allow the ring to rotate clockwise from where it currently sits. Then set the ring to the middle position of its new-found movement and mark it at mid-point of the range of movement of the plunger. Cut V slot. That way, the plunger won't slip out of engagement at full A or full R. The eccentric pin can then be used for fine tuning if necessary - to get the points to open just after the relevant flip point of the armature at full advance.
