and yes to answer your question, there are serial numbers on every removeable light engine and the body. For "dumb" heads like earlier generation L1 and L2s..I do not recall if there were a serial. It appears the serial might be used for product QC references, but I dunno for sure. I don't work for surefire or intend to work for them and I don't know PK's method of his madness so don't quote me on anything.
They do not have to be identical and no you can't tell the model from reading the serials to see if they are newer models. Given the density of surefires on the forum I think its safe to assume the serials are unique to each light, rather than batch numbers the way fenix [now 4evens] does.
I personally love the single mode E2L, the single stage version is known as the "2007 model" and is characterized by a single permanent marker stain on the anode.
User [I believe, but could be wrong] SelfBuilt made a runtime for it
A solid 9 Hours of runtime from 2xcr123As, the buck circuit really does a good job. A measure of current via DMM revealed the current draw at around 180ma, even less than the Gen II Inova X5 famously known as the battery vampire.
Since its a buck circuit, running it on single cell is not an option, the KX1 starts from there.
I forgot who did this but yes, the E2l will run on rechargeable 17670s, although given the runtime on primaries I have no idea why you would go this route.
I own three single stage E2Ls and is always on the lookout for more. Its a fantastic light with good balance between reach and spill, never gets hot or even warm, good light to keep on the car or in your BOB without the need for spare cells. It generally seen having welcoming asthesics, no combat bezels or anything "tactical" [or "tacti-kool"] about it. A good light for parents [or your better half] to use since there are no modes of selection and unenlightened folks often do not try to conserve runtime regardless of what light you hand them. From keychain lights to 3MCP spotlights, unenlightened folks will just leave it on pointing to the floor or somewhere unproductive and complain when the batteries die and they can't use it further.
I suppose your parents, kids, better-half, gf, bf, sylvester, fido, lizard, or goldfish you've chosen to entrust your light to exhibits such behavior, I suppose the protected 17670s might be a good route.
The E2L worked well for me, but due to its low power it doesn't fit everyone's choice for lights. If you don't find the E2L as apealing as it was described by yours truly above, I'll trade you a Surefire A2 [HA-WH, round] and a spare lamp for it, I'd offer you cash but I seem to be short of it. You can keep the tailcap
