I think you are looking at Post WW2 British numbers, such as 17BF88, which started in 1947. Wartime numbers were a letter followed by numbers, such as Z5344037. Wartime vehicles were renumbered in the late 1940s to the new number system, and this started at the end of the alphabet unless the vehicle had been rebuilt, in which case the first letter was an R, as in 12RA34. Please note that all Royal Navy vehicles were RN, as in 12RN34, however.
With wartime British numbering, the first letter referred to the size or type of vehicle (Z=15cwt truck, L=3 ton truck) and the number was one of a set issued to that contract number. For Example, my Ford WOT2 truck is numbered Z5344037, and the contract was for 5000 trucks, numbered Z5340144 to Z5345143.
Please note that Canadian Army vehicles had the same numbering system, but included an extra C before the number.
I hope this helps, but vehicle numbering is a bit complicated with the numbering system changing.