I know; I'm having to learn now.
Here is what I've determined (subject to correction):
1st gen X26= E Series. X00 serial numbers. Can be black, yellow or clear. All have the metal grip plates, all say 'X26' on the side. Analog spark gap output.
1st gen X26 for women and childrens = C Series. Only think they came in black. Same internals, same look, difference is programming. E (law) versions are 5 second per pull, on as long as you hold it down. C (citizen) is like 15 seconds stackable. (Idea is so that you light them up, drop the TASER, and book feet out of the situation).
2nd gen X26= P series. Looks different. No X26 on side. Digitally controlled spark output, current limited output.
Why does it matter?
P is different shape than E/C. E/C batteries will not work in P, and vice versa. X carts fit all X and M series.
E/C are no longer made, and I think started production in 2003 (you can look at the DANGERS-LAZAR label on the bottom for a created date on the E).
Dunno how long the P is going to be on the table, X2/waffle maker is what they are pushing now.
I've been carrying the C2 for years. I really like it, and only got this one because it was $165 out the door, shipped plus a holster and a fresh extra cart holding battery. I figured there would be a ton of X/M carts out there. Apparently I was wrong lol.
If you are having slow or diminished output, try taking an eraser to the two output points on the handle (that's what they call the unit, apparently). The used one I bought had a really weak output, I burnished those points up, and it got fat and loud quickly. Also, if it's an E, I'm betting cleaning the battery terminals in the handle and on the battery will improve the output rate.
Kinda mad, actually. Found online there was a way to be a citizen-only instructor completely online for $99. Apparently it was through a college, and is no longer offered. Still is offered, but it's more expensive, and you gotta truck out to AZ to take it. (shrugs)
I wish mine were two toned. I'd like a black handle, and a high visibility yellow snout. Keep it low profile on the belt, but make it look less like a snubby pistol from a distance when presented. Maybe I'll paint the carts when I get them...