2 as a Maglite thread change

I have an older, about 15 years old and a new 2 as a Maglite. I was going to change things up by changing the heads and tailcaps but the heads have different threads.

When did they make this change and why?

i know the i.d. changed on the newer lights with the battery size as the first character of the serial.
what does the serials start with on yours?

Sorry I meant to type as aaa