We're getting into what people often refer to as 'shunt wound' and 'series wound' dynamos here. Lucas standard for the E3Ls etc we have is 'shunt wound'. Many others (Miller for one) are 'series wound', with the field connected across F and D and the field strength being regulated on the 'live' side.
A DVR2 and some other regulators work only with so-called shunt-wound dynamos. JGs, and maybe this Wassell thing - when negative earth at least??? - need the dynamo wired the other way.
Most dynamos are capable of working either way, although there may be questions as to the resistance of field coils in some cases. A DVR2 needs a minimum field resistance of, I think, 2 ohms. All Lucas m/cycle dynamos, 3, 3.5 and 4.5 inch, have at least that.
I am guessing the MC45L Lucas instrument on a Beam is wired with the field between F and earth, but I don't know. If it isn't, then a DVR2 wouldn't work; if it is and a DVR2 doesn't work, then there is some other issue that I don't understand!