That doesn't really stack up KiwiGF. Coils are wound one by one (to the known spec in terms of gauge of wire, number of turns etc for all the common ones, some detective work required to figure out the 'recipe' for rarer things sometimes), and it isn't a big deal to go from, say, a K2F to a Magdyno MO1 or a BTH or whatever. All the decent trade winders out there operate much the same way. There are some differences in equipment used, as there are quite tasty CAD machines available these days, which speed things up, but it still comes down to a job by job approach for that side of it.
For the lotions and potions used to impregnate and cure the coils, yes, batches are normally done together - but '50 or 100' implies a factory-scale operation, whereas most of those who do this stuff are more in the cottage industry category.
One-offs are done all the time and every day - whether it be a pre WW1 Splitdorf, a Simms from a vintage commercial vehicle, or a tiddler from a small '20s 2-stroke (just to mention a few that have come through here lately and for which there was no problem in getting them rewound at reasonable prices).
The quality is a factor of who does the wind frankly. The old apprenticeships for winders were 5 years, by the end of which someone would be experienced in anything from large industrial motors and generators, down to the human-hair scale windings of HT coils for magnetos.
Considering the delicacy of the job, it is a tribute to all those who do the winding that the failure rate on rewound coils is pretty low. But as with all things, shit happens now and again, as often as not due to variable wire quality or insulating material problems. I and my Brightspark mate the late KenF must by now have fed way over 1000 HT coils through the channels we use for rewinding (neither of us was going to start self-apprenticing in late middle-age when there are fully-skilled people out there!) and of that number, there has been a tiny handful of failures, always rectified immediately by the coil winders. A K2F or MO1 is a standard job for most of these people, and they do them quickly and well, so we shouldn't have any great problems in sourcing good quality refurbishments for as long as these folk keep going.
That said, there are rather less good efforts out there too, which don't pass the basic coil tests imposed by Lucas (as found in their workshop instructions). Comes back to 'who does the job'.
This does lead to the question of 'what next'? Most people doing this stuff are not young (60+ is common). Most people's offspring can think of less tricky ways to earn a crust. Beezermac spoke recently and very eloquently of the difficulty of getting a work/life balance, and it's the same problem for a lot of people. In another few years we shall have to have fresh solutions to the problem unless new blood comes into the arena. We've had 20 or 30 years of relative comfort, which followed the difficult period where NOS Lucas parts became thin on the ground and there weren't many folk offering rebuild services. That era is coming to an end.
One interesting discussion in which I am engaged is for the possible manufacture of brand new armatures, with new CNC-machined end cheeks, with new laminates and coils: ie, NEW, so no oversized air-gaps due to several-times-machined parts, no bent drive ends, no damaged tapers, no messed up threads and mismatched parts. But these are not trivial things to do, requiring investment and time. And it also has to be looked at in the context of some of the competing ignition options that are becoming popular. While we who want to stick with 'proper' magnetos are all still able to recycle only faintly-mullered parts to keep going, the need may not be desperate - but the day will come. Many BSA owners complain that their ATDs turn out-of-true on the end of bent shafts, that the ATD or pinion doesn't fit where it should on the taper, etc. The idea of having completely new bits available is appealing. So is the parallel idea of making brand new brass-type contact breaker backplates of a better quality than the steel-backed type that is currently available. Again, it's a question of time and money and the waning availability of serviceable bits. We're a notoriously tight-wad group of people sometimes, thinking often it terms of what a factory price-list from the '50s looked like. When Lucas was brow-beaten to supply whole magnetos for not much more than a pound. BTH refused - and although they made good mags, how many KC1/2s are there compared to Lucas K1/2Fs? Norralot.
I've strayed off the initial topic here a bit, but the main point is that 'any job can be done, and done properly' and should be affordable while the skills last. When the skills have, literally, died away or retired, what then? Electronic options are luckily there, but we need also to be thinking about revitalising the core competence of maintaining and mending magnetos, and ensuring that parts remain available, if people want to continue with what the makers intended.
