The chassis plate is a series of numbers and letters that are stamped on a metal plate that is riveted to the body of the car. This plate is located in different places across all mini generations and is used to identify the model year, assembly plant, and sequential build number of the car. The chassis plate is also known as the VIN plate, VIN tag, or VIN plate depending on the generation.
For the year range selected, here is an example of what a chassis code could look like.
A-A2S7L-###A
Choose the options to decode your chassis code. Please note that some fields will be disabled if those positions do not exist on your model year chassic code.
X X X X X
The first car at Longbridge was number 101, as was the first at Cowley. From then on, each factory continued with their own number sequence regardless of the model; e.g., Saloon, Van, etc. In other words, each model did not start at 101 and maintain its own sequence. Saloons and Vans, etc., are mixed in the number sequence along with the Cooper and Cooper S models
This seems to be used inconsistently on English-built cars, and it is understood that the car was built at the “normal” factory if the letter is left off; e.g., an Austin, Riley or Wolseley at Longbridge may or may not have an "A" after the sequence number. Yes, there were Austin Minis built at the Morris plant in Cowley and Morris Minis built at the Austin plant in Longbridge. Note that all English-built Cooper and Cooper S models (Austin and Morris) as well as Riley Elf and Wolseley Hornet models were built at the Longbridge, Austin plant
Please note the above details were pulled from Mini Mania's technical articles. More Details can be found here: Chassis Code Technical Articles