Have you tried running it clockwise?
Why not try running it as a motor off a battery to see which way it actually spins? D&F bridged, battery one lead to the bridge and t'other to the dyn body - according to whatever earth you are set for- and see. (That'll incidentally confirm the polarity - which earth - for you.)
If it goes the wrong way, change the field wires over or swap the brush connections to reverse it as already stated. (Not both.)
If it's producing 7v in some circs, I kind of doubt the rotation is actually wrong (unless your 7v is actually 0.7v, which is about what I'd expect in those circs). Also, I'd have thought Drags would have got it right if you told them what engine it was going on.
When testing with a bulb, it's often best to get the dyn spinning and cut in before hooking 50W up. 50W/12v and a cold globe will take a fair bit of current to fire up (like maybe 8 amps+), and the thing won't necessarily be inclined to produce that much from a standing start.
The only other 'funny' would be if Drags had built the dyn for earth-side regulation, rather than the normal Lucas live side. That would be visible because the two field coil wires under the lid would be hooked between D and F, not D and Earth. Highly unlikely . . . but some regulators (JG for example) work that way and need it, and it was standard on Miller and most other non-Lucas equipment.
