Nice early BSA with the oil pressure indicator and also a dipswitch. A previous post said dipswitch's weren't on wd bikes?? It looks a very old picture to me. I have a dipswitch on mine, can I now have a go when the rivet counters kick off!!
I think you'll find the timing cover plug is an early (civy) feature, along with lots of others. Valanced guard, large tank, 8" headlamp, oil tell tale, rubber parts, ally tappet cover etc. All to be replaced by cheaper and more utilitarian WO specifications. Ron
The clue is in the contract number roughly painted on the front mudguard - C5610 delivered from February 1940. This one doesn't look completely new but presumably it still only has the census number on the drive side and some machines from this contract began to display them on the number plates.
I'd say that it looks bang on for a 1940 WM20 produced once the MT110 had been standardised.
DME Circular B.141 dated 12/5/1941 detailed the "Fitting of new headlamp mask and removal of dipper switch" so dip switches seem to have remained present until mid 1941 on bikes still using the paper blackout mask at least, but they may have been discontinued earlier on new production.