There has been much written about all the Minis in the USA with confusing heritage. 
Knowing what year your car is may not be that important until it comes to trying to be
sure you get the parts you are looking for. 
The Mini was first produced in 1959 and continues thru the year 2000.
While all the years may look very similar to the newcomer, there are indeed a number
of model changes that make parts interchange a real mine field.

One of the more comprehensive reviews on “How to ID your Mini” can be found here:


http://www.minimania.com/matrix/

One of the quick visual methods is to simply inspect the boot/trunk lock assembly. 
As you open the boot and look at the top edge of the assembly you will in many cases
find a 4 digit number stamped into the edge.  This stamping can be very faint and at time very difficult to see.
The numbers give us the week and month of production of the assembly.
It is true in most cases that these assemblies have not been changed since the car was
produced and thus also tell us pretty closely when the Mini was assembled. 
In the picture shown below you can see the number 33 80, thus this tells us the Mini was assembled during the 33rd week of 1980.

Click for larger Image