Great bit of info B_UK. Love wooden threads.

Not much help for you, but others that are interested in a diversified range of gauges, the Beall Wood threader kit (now sold by Lee Valley), can work out cheaper (at least here in Aussieland) than the equivalent set of the box threaders pictured. 
The down side is that you need a trimmer (which most have)... I have a Bosch trimmer with micro adjustment (which I think is most useful) dedicated to the jig.

Cheap and woodwortking in the same sentenc is an oxymoron!