It probably doesn't mean anything officially , I have seen all sorts of numbers and letters stamped on the end of mini engine numbers .It was quite common practice (although illegal) Usually If they don't have a matching engine number some people just machine the block and re-stamp the number that is correct for the car , and add another letter or number to the end , just in case there is another vehicle registered with the original engine number
Edit : The exception is probably blocks with an 'S' on the end of the number , which apparently means the block was porous and was sleeved at the factory