The letters before the serial numbers are a prefix. It is part of the serial number. They would start with 00001 and then work up to 99999. If they needed more numbers, the prefix comes in, A00001-A99999, then B00001-B99999 and so on. Some would start with the A prefix, just depends on the country. Some countries also assign the prefix blocks to certain times or manufacturers so it may not mean that many rifles were produced prior. It isn't real helpful generally to tell when the rifle was made.