Still no luck with wiring diag for 7B12 but 'Delco Don' (contacted through Sparks & Arcs) has been kind enough to email me details of 4B12. The step from 400W to 750W is quite a large one and the two sets are clearly somewhat different, but I think it has given me enough of a idea of how Delco did things to enable me to piece together how it should all work.
A few things are puzzling me though and perhaps those of you with more experience of electrical gear can help.
1) The 'hot' side of the ignition coil appears to be connected to the starting lead of the dynostart. Thus it is energised directly from the battery only when the starter button is pressed, once the engine has started it must pick up its power through the dynostart somehow. Does this sound right?
2) As well as the heavy starting and charging cables coming from the dynostart, there is a third lighter lead which is connected via a preset rheostat to the terminal board. I took this to be the shunt field winding, however the rheostat hardly looks capable of carrying the full field current being only about 1/4" in diameter. The 400W version has the shunt field connected directly across the brushes (which is I think why these sets should not be run without a battery). Could the smaller lead on the 750W set be some sort of control or subsidiary field coil.
3) The dynostart is a four brush machine, but these are not spaced equally around the commutator but over only about 180 degrees. I don't know how the instrument is wired internally, I am treating it as a 'black box' with three leads and a ground connection, but I assume that there will be a shunt field winding for charging and a series winding for starting. How would these relate to the four brushes?