The earlier the Carrera the less likely it is to have a signed crown. The first execution Dato 45s introduced in 1966 don't have them, and it pretty much looks like any other Carrera before that shouldn't either. Once we get to the Carrera 30s being introduced circa 1969, signed crowns have become the norm. The changeover thus seems to happen at some point between 1967 and 1969 - it's not a hard and fast change, though, and crowns are relatively easily changed.

A good rule of thumb is not to be put off an early Carrera just because the crown isn't signed - that's how they were originally.