After eight years of production, the serial numbers reached the 105000 mark. In 1965 they were at 107000 and in 1967 they reached 110000. We can notice that in the early years less than 1000 exemplars were manufactured annually. Towards the end of the Sixties more than 1000 exemplars were made each year. Based on this numbers, 1957 seems to be a reasonable value.I don't know how fast they were churned out but between 54 and 57 is a good guess as Balogh stated.