U2 ultra serial number question

I was cruising through ebay this morning and noticed that one of the many U2 ultras and noticed that the serial number is now starting with B. Does anyone know when surefire started to serialize U2 ultra lights with a B series serial number (it has the new SSC LED). What is strange is I have one of the newer U2 ultras that has the SSC LED and it still has the A serial number

Did you buy your SSC U2 from the MP? Alot of people bought the "A" Models for the body (the ones that fit the 18650) and resold them with B-Bodies...so, alot of U2's on the MP have heads that don'r necessarily belong to the body (or didn't when they left SF).

Hope this somewhat answers your questions. The A-Body models with-out the sleeves in the battery tubes fetch a premium price on the MP as well...

I have two A serial number P4 U2 lights and neither can fit an 18650. They would have to be bored out to fit 18mm diam cells. They barely fit 17mm cells (hard to get the AW17670 cell out again). Regardless, my measurements suggest that if you try to run a stock P4 U2 with 1x17670 or 1x18650, you won't get close to full output. I estimate that you need about 4.5V input to reach full regulation in Max mode for the U2. At 3.7V nominal of a Li-ion, you might be delivering 0.3A drive current to the LED. So I'm not sure why folks are paying a premium for something that gives them less performance, unless they use them in an old boost version Lux V U2 with a 2S2P MC-E transplant or they mod the P4 U2 with a low Vf XM-L. But don't the old Lux V U2s already have a tube that can take an 18650?

With a P4 U2, it is probably easier and cheaper to use 2x16340 if you want rechargeable cells. The total energy content isn't great, but it's not terribly bad -- 7.4V*0.5Ah = 3.7Wh. That's probably good for about an hour run time on Max. An 18650 could have (give or take) 3.7V*2.5Ah = 9.3Wh, or a factor of about 2.5X greater than for 2x16340.

No, I am referring to a brand new in box SF U2 ultra on ebay that has a B series body tube. The one I nabbed off ebay about a year ago was an A series body tube, so I must've gotten lucky. It seems like there were several iterations on the A series tubes as in there were 2 different "U2 digital ultra" etchings. If I recall some have the "fancy" Digital U2 ultra etch, while others just say U2 ultra.