Hi Ian,
Yes your BTH MD2 T8 is another of the 180 degree twin magnetos for stationary engines, with shielded ignition and thumbscrew a/r. The example in the photo below is from 1950, and uses the heavier KD2 casting with 10mm machined off the bottom by BTH themselves.
I started researching BTH magnetos about 7 years ago and found nothing much at all; even the "experts" I contacted didn't have much info. So I gathered info from 550 BTH magnetos (in the shed, on bikes, on the internet...) and built these into a spreadsheet of serial numbers, models, forms and features. Plus all the info I gathered from other sources. Of the 550 mags in the spreadsheet, only 2 or 3 don't fit perfectly into the pattern (maybe mis-stamps?), so I reckon I can date to the month any BTH MC, KC, MD, KD, MH or KH magneto from 1929 into the 1960s, by which time the mags were branded AEI. Unfortunately the earlier horse-shoe-magnet K and M magnetos don't fit into the same series of numbers, so I know precisely nothing about them!
I did all this because I wanted the answer to one question: what magneto should be fitted to my 1932 Works Rudge? It was certainly a BTH TT Magneto (even though Rudge were fitting ML magnetos on all their bikes at the time), but which model and form, and how would I know if it was the right date? Anyway, the bike was raced in the TT in June 1932, and after seven years of research and searching I have just acquired (for a staggering amount of money from a seller who knew nothing about it) an MC1 E6 TT Magneto with a build date of... June 1932. It is the earliest TT Magneto I have seen (they were introduced for the TT in 1931) and is extremely close to the correct specification for the bike. I am thrilled, even though most people wouldn't notice if I fitted a KD1 C6 TT Magneto off a mid-1950s BSA Gold Star!
Cheers
Leon