BruceM,
Referring to the top diagram in Gary's post (fig 4), the main field appears to be connected to the brushes on the "D.C. Arm". I believe this is 4 brushes that contact a segmented armature and provides the current to power the main field. The variable resistor there adjusts the no load voltage. These brushes appear to be wired in parallel, the two vertical bushes labelled "+" and the two horizontal brushes labelled "-". These same brushes carry the current for starting (which also goes thru the series field winding), as well as provide current for charging the battery. Note that starting the unit should provide an automatic field flash - the initial magnetic field to get things going.
The axillary field is connected in series with the load via the bridge rectifier. This provides an increase in the field with increasing load current, which should help maintain a near constant voltage with varying load. The "diverter" resistor bypasses some of the load current, thus provides adjustment for voltage at load. Note that smaller resistance here provides for LESS boost at load.