A28. 14x1 female spigot back
Links to photos of examples:
Many of the original valves had poorly made cast nuts. The nuts were too big and also weak. When fully tightened they would strip. Some have good nuts, but after 35 years the rubber seal disk inside is brittle and deformed. All of the mopeds listed that came with a spigot-back gas valve, can take a less expensive spigot down or spigot left valve.
The A28a valve has a 7.7 mm spigot, for 1/4″ (6.3mm) fuel hose.
Honda Design: Note that the little arrow on the handle points opposite to the handle. So off is arrow left (handle right), on is arrow up (handle down), reserve is arrow down (handle up).