It's definitely a JG 12v box, and it almost certainly required the mods on the internal wiring unless they did 2 versions back then. I'm quoting word for word and/or paraphrasing here from the original (photo-copied type-written!) instructions that came with one I bought in the 1980s and which still works (although I've replaced it with a DVR2 as I think they're better).
"For positive earth only. The green wire is to Dynamo F; the blue to dynamo D; the black goes to ammeter or switch etc and the box has to be well earthed by its studs. Have to scrape paint for that.
The dynamo's wiring has to be configured to generate -ve voltage. Wire the field and brushes as follows:
* the nearer brush AND one field coil lead attach to the D terminal;
* the other field coil winding goes to the F terminal;
* the second brush simply goes to earth at the little screw on the dynamo."
(The difference between this and the original wiring is that the second field coil lead goes to the D terminal rather than to earth - ie the field is connected F to D, not F to earth.)
"To test the earth (polarity), first connect a wire from F to earth, then connect (they say a 6v) battery live (negative) to D and battery positive to earth. It should motor. If it motors the wrong way disconnect battery and swap field coil leads over."
(This by contrast with the usual 'Bridge F and D, run them to batt live and the dynamo body to batt +'.)
As Trev said, motoring isn't a perfect test - but it's a good start.
"There should be up to 14v at the blue lead as the revs rise" according to the data sheet in front of me.
Interestingly, the JG paperwork also blandly states 'it will boost the output power to twice the original specification'. Well, it won't, but you do get some increase . . . I happily ran a 60/55W halogen round central London for years with mine, and it just about managed it.
The box itself is an OK gizmo. Sometimes it needed a handful of rpm to get the thing to kick in from cold, it allowed a discharge into the field at tickover (same as most mechanical boxes do) but it generally maintained batteries well for those years. However, it's not as good as a DVR2, which kicks in better at 12v, allows no discharge into the field at tickover, and is smaller and easier to conceal for those who prefer to do that. Maybe the modern JG boxes are smaller, I don't know.
You should find the dynamo wiring change has been made already, and all the kit should be 12v. Unless someone's really made a dog's dinner of everything!
