Just a quick clarification.
I've got a few 1969's listed with only 4 digits, all of the 1970's are 5 digits starting with 0 or 1.
I assume you are talking Y bikes here. I really don't know if I have seen one starting with a zero. I would have thought they would look out of place in the 1967 despatch logs as well.
Is it possible we are talking cross-purposes.
I have been saying all the normal 1970 numbers are 5 digits, to the extent that a zero is used at the start if necessary.
I am not sure about the Y bikes. The only two I have pics of are 4 digits without the zero... which then makes Mike's engine acceptable. Albeit 1900 is a very low number for a Y bike.