24.0 and 23.8 are commonly used to describe the same handlebar diameter, for where the shifters and brake levers fit. Interruptor levers are available for 23.8/24.0. Sometimes 26.0 and 31.8 levers can be used with a shim to 24.0. The Salsa levers are like that. Tektro makes interruptor levers in all 3 sizes.