May, 1955 for the Model 1 March, 1957 for the plastic wheeled Model 2 (serial c.75,000) September, 1957 for the Automatic and 2A March, 1958 for the Special
Not sure on the Model 3 but assuming it was also around March, 1957.
Adding in patent dates etc would also give some insight into the development cycle.
Think I need to start working backwards from the Model 1 for some further clarity on the chronology. The work done by the guys at the start of this thread has been an awesome help!
