If it is a reversing dynostarter it will not earth through the engine but through the black box. So first check is that the dyynostarter does not have a common contact with the engine otherwise when you connect the battery you will damage something. So there is the clue to work out the cabling in the absence of a wiring diagram, though sometimes the black box loom is illustrated in the lid of the black box.
Anyway the test for forward is connect the thick black wires B2/D+ to HE/92L and put B1/92+ on to the negative terminal of the battery and A/30b, the power cable from the regulator, to positive. This should rotate the engine forwards.
Leave A/30b where it is and swap B1/92+ into connection direct to HE/92L and place B2/D+ onto the negative terminal of the battery and the unit should now rotate in reverse, mimicking what the change over switch does.
The small black, UB, and red, UR, wires with are the points so should form no part of this circuit. The other thinner black wire with the removable tabbed terminal, DF, is the charging control wire back to the regulator. Lastly there is an earth wire purely as a convenient way to ensure a good common earth between the black box and engine when the units are in place. The battery is normally wired with earth to where that black box wire is connected to the engine on the cars I have owned using these type dynostarters. That does not mean it will always be the case though as I have not run all the odd Villiers machines up.
Trust this helps.
