The serial number was greater than 50 and the last serial number given in this system was 91. The letter is followed by a serial number. There was a lot of variation in the way points were used in the two above mentioned systems. Finaly on 1 June 1927 a new system, which consists of a type code and a serial number and which is still in use today, was introduced. For example MA-24, where MA is a code for Martinsyde F.4 Buzzard.
More recent jets seem to follow similar pattern.
Another way was to start a fresh set of serial numbers from 1, this was often used for training aircraft and more generally during and after the war. Go to Military aircraft markings 1918-39 for a complete list of markings.