Thorn Adjustable Tandem Stoker Stem 28 degree

28.6mm clamp for seatpost (shims can be used to reduce this for smaller diameters)
25.4mm clamp for bars
Adjustable section in 170mm long, stem clamp can fit above or below the "tube" section.
The "tube" section is also 28.6mm so any modern 1 1/8in ahead stem can be fitted instead to prove an alternative position.