On the Cropmaster, 25 & 30 types there were no numbers as such stamped in the castings, these were date coded raised letters and numbers, normally found under the gearbox housing, it was more normal for the numbers to be stamped into the plate on the dash panel, if this plate is missing it is hard to determine the serial number.
Stamping of the serial number into the casting was introduced with the introduction of the 900 series. The number was stamped into the horizontal part of the front casting (tombstone), just behind where the radiator bolts down, or just rearward of the bonnet side panel. This procedure was adopted for all Implematic and Selectamatic, 9,12, & 14 series tractors. On the 90 series tractors the serial number is stamped on the top of the engine chassis casting below the lift pump. For engine designation and numbers, all 3 cylinder numbers were stamped on the block about mid way up behind the drive connection for the rev counter cable. 4 cylinder models were stamped up the front of the block just under the dynamo/alternator. The first set, for example, AK/4, AD/4 denoted the type of engine, 3 cylinders were 55/1. The numbers after were the actual engine number.
Thanks to Powerrabbitt for the above information.