The early KX1 and KX2's were single stage.
My dual stage KX2 has a serial number starting with a B. From some other pictures I've seen on this forum, the single stage KX2 has a serial number starting with an A, the dual stage KX2 serial starts with a B and the single stage KX2C has a serial starting with C.
I don't know if the serial number designation is coincidence or if SureFire planned it that way. If it was planned, I'd be led to believe that the KX1 would also have an A or B serial indicating which model it is.
EDITED TO ADD: I should note that the serial number you'd want to check would be the number on the KX1 head rather than the number on the body.
It is probably single stage. =\
I like my single stage E1L. It's not too bright, not too dim, and it's a pleasing tint. I use mine for working on my car.
Hi J-Na806 and :welcome:
Please find herebelow side by side the pics of the two packings:
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On the left (2007) is the last single stage version (CREE LED), on the right the current two stage type. I believe it is impossible to tell the flashlights one from the other except by switching them on. The packages on the contrary are easy to check. :)