Hi,
Geoffrey Butler Serial Numbers.

I worked for Geoffrey Butler Cycles on Saturdays and School Holidays between 1964 to 1968.
Whilst I was their Burt Sullivan allocated the numbers and they were usually stamped on the underside of the bottom bracket and of the steerer of the front forks using the following algorithym:

Sequence number for the month 1-xx, Month number (e.g. 7 for July) Last two year digits.
For example: The first frame of September 1968 has the serial number 1968.

Hence you are able to identify when the frame was built prior painting from the serial number.

Chris