Uh, would you please explain what a underslung demultiplicator modification is, and how it works?

In this photo you can see the home-made bar end shift lever on my 40's Fothergill:

The lever is held by a heavily modified Suntour bar end shifter body, which had been broken previously.

In the same photo you can see a handlebar bag that is also home made, custom tailored, etc.

You can also see the 1930's (?) Centralpull brake, for which I had to fabricate part of the clamp that holds it to the fork. The part that isn't rusty, that's the part I made.

The shifter, shown above, operates a Resilion derailleur that shifts the chain between two cogs on a Sturmey Archer dyno-4-four hub.

Of course the derailleur isn't home-made.

You can't really see it in the photo, but the tail light is attached to a home-made bracket. There's a similar bracket on the fork.