not sure about the P and K, but the 4 indicates the led has a slight blue tint. about the serial number, arc probably doesn't ship out the lights in order of serial number, and sometimes older stocks of bodies lie around before being assembled into completely lights. the first arc LSH i got had a serial number of 848, but the most recent one i have has the serial number 016 /ubbthreads/images/graemlins/smile.gif