Looking back through posts on the forum over the years, there are quite a number of questions about the identity and date of BTH magnetos for Douglases. Many queries are still unanswered.
Starting from the EW (or maybe even some CWs?) Douglas mostly used the "early" style of BTH magneto, which featured a horseshoe magnet made from a tungsten alloy steel - for example the M2-W2 usually seen on the EW or the M2-AD1 and M2-AE1 used on the bigger twins.
From 1929 BTH introduced a new range of smaller, lighter magnetos (the MC and KC range) using 15% cobalt steel magnets, then from 1934 the MD and KD magnetos with ALNI (aluminium nickel) magnets, and finally the MH and KH magnetos with ALCOMAX III magnets. All these later magnetos have an id plate, with the model (eg MC1,KH2...), the "form" (eg B4, C6, AC-9), and a serial number (eg 7K551392). Between 1929 and 1969 (by which time the magnetos were branded AEI), all these numbers make sense and magnetos can be dated to the year and month pretty easily.
By contrast, the numbering on the early-style magnetos is a mess. I think M and K series magnetos with horseshoe magnets were available from the early 1920s (maybe even during WW1) until at least 1946, so there's plenty of scope to get mixed up with dating and correct usage.
Anyway, I think I'm in a position to answer questions about the various BTH magnetos, in particular about dating. I appreciate it's a pretty fringe topic so I won't be hurt if no one else cares!
I'd also be really interested to know if there was any special relationship between BTH and Douglas: certainly in the 1920s magnetos numbers in the 800000s seemed to have been set aside for Douglas of various dates, while the other BTH magnetos of the era carried on from 900000, so that the magneto M1-B2 7L927423 seems to have been built a couple of years before the Douglas BTH magneto M2-AE1 9E849795. Not to mention the 1926 M2-W2 magnetos for EWs with numbers in the range 10000-20000 at a time other BTH magnetos had serial numbers around 600000. Nothing like this happens with the numbers in later magnetos!
Cheers
Leon