I am looking for 349 tires with tan sidewalls. That mimics tubulars
But they are very hard to find. Most of the big brands of utility tires are quite boring and just manufacture black tires for 349 mm. Biggest margin for profit.
The only model so far is (and that is also quite rare with tan sidewalls):
Primo Comet 37 x 349 mm (16 x 1 3/8")
Brompton them selfs shows very few alternatives.