What threads are used on Avid BB5 barrel adjusters?
They did indeed turn out to be M7 (1 mm pitch).
Nuts are fairly easy to get, but at least the locknuts (half nuts) I bought take an 11 mm spanner.   None of my multitools, or any I can find online, have this size spanner built in; even my workshop spanner set omits it, but in over 20 years of building and maintaining hardware for work and play, this is probably the first time I've handled M7 parts.