That is odd. For actually a long time, M5x1.0 has been the popular pitch for bike body equipment. Being that it is a Cannondale, I can actually
assure that it is a metric measurement. So, between M5x1.0 and M6x1.25, there is no such thing as a different measurement which I do know of, and I’ve 20+ years as an engineer in manufacturing. Maybe the opening obtained stripped-out and any individual grabbed the closest faucet measurement to the opening? With 1″=25.4mm, figuring that an M5 is roughly .20″, strive perhaps a 7/16″x18tpi?. Perhaps it was REALLY stripped-out, they usually tapped it to a 1/4″-20tpi (which occurs to be a VERY widespread measurement). Tell us what you discover….
I see on charts that an M5.5×0.5 thread exists someplace on the planet, however I’ve by no means seen one in MY life.