There doesn't seem to be any form of regulation on the car only an electronic cut out. I started the car today and tried measuring things with my electronic multimeter. The voltage on D seems to be all over the place and the field voltage was tiny! on 1/2 charge the current was not enough to swap from discharge to charge but on full charge there was a charging current. It does seem odd not to have a regulator I might sort something on that or else I will fry my battery I reckon.
This article does describe the 3 settings I have found on my switch being used on the A7 (last section). (but not the switch itself) I don't feel confident that my dynamo has been re-wound so I will stick with the two settings for now so I don't put the full 12V on the field windings.
As mentioned, until someone publishes a circuit, I am not familiar with the pre RP arrangement, and it is a pain to fathom from wiring diagrams.
At the risk of prolonging this, the direct links shown on the switch in Fig 1 of the article introduce confusion.
Whilst the article may have come out of A7CA mag, as far as I know it does not apply exactly to any Seven.
Digital meters do not work well in a car ignition environment, and they can get confused by erratic and noisy voltages.
Much depends on whether your system is operating as 3 or 2 brush.
Allowing for meter lead resistance, the resistance field terminal to 3rd brush (or earth if none) should indicate if wound for 12v or not. I measure at about 1.5 ohm for 2 pole models originally but others quote somewhat more.
From playing on the bench the field seems to tolerate considerable current without getting very hot; this may explain why many somewhat random 12v conversions survive. However in the car has a lot of other heat to contend with.