its around 1927 BTH maybe off EW350/600
Exactly - March 1927, and M2 W2 is standard EW issue. Interestingly BTH seem to have set aside the numbers 82xxxx onwards solely for the use of Douglas, while their other magnetos carried on with serial numbers from 90xxxx. So the trap is that this magneto M2 W2 7C 835353 is a March 1927 magneto, while similar numbers M2 AD4 7C 86xxxx would be a March 1937 magneto. 7C on a BTH magneto could also mean March 1947, March 1957, or even March 1967.
I'm not sure why BTH put these special numbers on magnetos for Douglas - magnetos supplied to everyone else fall into simple numerical order.
Cheers
Leon