They are an obscure thread which is CEI (Cycle Engineers Institute), a forerunner of the BSCY ( British Standard Cycle Thread).
When the BSCY threads were formulated 26 TPI was decided upon, but in diameters above 1/2″ they kept an option of 20TPI from the old CEI threads. Very rarely I have seen Scott Flyer/Rep/Sprint rear wheel spindles at 26TPI, but they are usually 20 TPI. Lots of tap and die specialists who advertise in the classic magazines, for instance Tracy Tools, can supply 20TPI taps and dies.Still larger diameter threads, such as headstocks, also throw up some oddities, so don’t rely on anything being the standard 26TPI Cycle thread !
P.S. Similar comments apply to the three engine mounting studs/bolts with 20TPI being usual, but 26TPI occasionally cropping up, ditto the hollow bolts holding the rear of the gearbox tray to the frame lugs.
Further to my previous reply, I have found a very good BSCY thread list on the https://www.motalia.com website. This lists six different diameters with a 20TPI alternative to the 26TPI. These are as follows:- 7/16,1/2,9/16,5/8,11/16, and 3/4″. Hope this helps.