I took your advice Charlie and contacted MERL. Unfortunately, they were not able to find any information on my Allen Mayfield scythe. It could be that I have read the serial number wrong, as the plate is badly corroded. If anyone else owns such a machine, could you confirm if the format of my serial number (two letters followed by 4 digits) is likely to be correct?
On the plus side, they dated my Allen scythe to 07/05/1969, being delivered to a Mountain & Sons Ltd.