I own a couple of Sega Mega Drive consoles and i'm wondering how the serial number on them works. One of them has a serial number something like "40e 13000" and another has something like "40f 38000". I assume that the 40e is an older console, but how far down does this serial number go? Does i.e 35a excist? And how high does the number goes before the letter changes, does i.e 40e 99999 excist?

I also have another console that has a full number, something like 12515162, no 40e or anything. I assume that this is a newer model since it doesnt have an EXT port on the back. What was this used for anyway? To connect Sega CD model 1?