I may be able to help, as I have a similar or identical regulator plus some bumf on adjusting it. I have had a preliminary look but it does not seem to state the resistance of the precision resistors that are open circuit in your unit. It gives the regulator type as VS/75/18M and the coil current as 0.265A.
Clearly the genset is not a searchlight set, they were all DC. There were half a dozen models of Lister JP4 powered AC sets but if your friend's plant has a Metvick alternator I suspect it might be an Air Ministry 20kVA Mark II, as most Lister sets had Mawdsley's alts due to the connections between the two firms, as well as Isenthal regulators. The 20kVA had the Tonum regulator and a rather complex electrical system, due to the spec of the electrics for its particular application.
It's worth noting that I have had trouble with the carbon piles in these before and never got really satisfactory results from my reg. It is theoretically on the shelf for a rainy day, along with another that I am trying to acquire for parts. If my resistors are still OK, I can measure them, although I wonder whether they were matched to the torque-motor coils.
First, though, are your plant and electrics anything like this (less the trailer etc):
http://electrokinetica.org/d3/4/1.php