After lots of checking, it seems likely that the contact in the dipper for main is not conducting fully, whether corrosion or just bad contact. The dipper and the main beam flasher are probably supplied by two different sources, as they still are in the later EFI bikes. It might be said that the main beam flasher had a lower resistance, but this does not explain why the low high switch gives a significant difference in the light output. I take it the bulb is ok as it seems to light with the main beam flasher operating it, dipped works ok, via the main switch, leaving just the main switch contact between incoming power and the main beam out.
Of course, it's also an Enfield, so may have a few scientific laws that haven't been discovered yet.
Sorry, best I can come up with at the moment, don't have a electrics diagram to cover your year, but the link below takes you to a wallchart giving explanations of every electrical circuit in a (1995) iron bullet, along with a colour diagram.
Bullet electrics
or see the rest of the stuff including modern efi wiring...
Scaly's junk
Again, my best shot at the moment... if possible, swap the two wires between high and low switch, and check again, see if the low beam goes dim...
