.02, the likelihood of finding triple butted 2.3/1.8/2.0, or 2.18/1.8/2.0,
in a short length
for building a 406 20" wheel is pretty unlikely
*(unless ordering thousands)
Rear of Bromptons use a 2.5mm thick straight gauge spoke ,
28 in a 349 wheel-rim.
Front they use lighter gauge spokes, same count.
*I bought a (new XS) Schmidt Dynohub,
it uses a custom ordered straight pull DB spoke ,
made just for their wheel building section.
they would get that quantity..
...