I believe the boss was intended for a drain plug but was not proceeded with as it was thought too vulnerable and likely to be swiped off by the nearest kerb.I have 3 twins,
/2646 has a boss on the underside of the crankcase part of the primary, it has been drilled and tapped and has a plug fitted.
/166 has the boss but no hole or plug
/1997 also has the boss, and before engine assembly starts (soon I hope) it will also be drilled and fitted with a drain plug.