But after a bit of fiddling this morning with some heavy duty aluminum foil, it turned out NOT to be the switch, but the fact that there was quite a bit of corrosion at one of the contact points. I thought I'd cleaned it enough to allow electricity to flow, but I guess not. I bypassed the corrosion with a strip of foil taped to the outer casing and - presto - lights on and the switch works fine.
Now, let me say, the torch uses "D" batteries (which I've pretty much tried to eliminate from the house in favor of "AA" and "AAA" wherever possible) and they are HEAVY. The output of the lamp is not very good compared to modern LED systems. But, DAMN, does it look sweet up there!
Here are some closer pictures
Here is a head on view. Note the face of the lamp is glass in a chrome ring. There is an "eyeshade" over the top. This ring/glass assembly is held onto the body of the lamp by two flip clamps on the right and left side.
Here are side and back views. Note again, the back cover is held in place by another larger flip clamp. The square thing on the back is a slider space that goes over the bracket on the head tube of the Twenty. It just slides on and off using the little handle!
From the top down, you can see the "big red switch." It takes a bit of force to move it, but there is a satisfying click when it goes into place.