"Would you please explain what a underslung demultiplicator modification is, and how it works? "


Demultiplier (Démultiplicateur in French)
A bell-crank-like device formerly marketed by Simplex as an accessory to their bar end shifters. It attached to the seat tube just above the bottom bracket. It served the function of a cable guide, but had two separate cables attached to it. The cables attached to a pivoting part at different radii, so the mechanical advantage was altered by this unit. This made it possible to shift reasonably well with the barend shifters, even though there was a stone-age Simplex derailer at the other end.
The cable from the shifter connects to point "1", acting on radius "A".
The cable to the derailer connects to point "2" acting on radius "B".
the cable travel at the upper section is increased by the ratio A/B. Cable tension on the segment between the shifter and the demultiplier is reduced by the ratio B/A.


Well this essentially explains the demultiplicator's form & function. I've just had to route mine underneath the tandem, rather than alongside the bottom of the seat tube. I'll be needing it for all the length between the rear gears and a Simplex retrofriction bar end lever. It's an overcomplicated bit of brand collaboration, but I've had fun cobbling this parts bin puzzler together.

Love that Fothergill! I've seen a grand ol' wooden-knobbed bar end lever that would look right at home on it. Are those shallow Lauterwasser/Highgate bars or inverted North Roads?
I've also been wanting to upgrade some Luxor Le Martele lights with LED, I'm sure I'll have some questions.
That brake arm light bracket is an elegant solution, looks the right part. Sure wish my touring tandem had the internal wiring option... I'm a fan of that useful little rear mudguard fastening, and the repurposed phone cord!