I belive one of the letters stands for the factory where the bike was built. I thought it was K for Kirby, but I'm just guessing. I have an email from Moultonguy (I believe) that details the layout of the serial number, but I'm on vacation and not on my machine, so I don't have it handy. When I get back home next week I'll forward it along or perhaps someone herewill post the info.
Brian