They may have been some LC cases in 66, though the C would always have meant close ratio and clubman.
the 'C' was intended to indicate the close ratio gear cluster. The Lightning Clubman would have come with the CR gear cluster, so being stamped LC may just be a happy coincidence.
I’ve actually Amended my text to “not clubman”(I should proof read before clicking post)
A bit like the DC casings, all clubmans (least from 65) had DC motors, but not all DC motors went into Lightning clubmans.
Bikes with close ratio boxes could be ordered (or at least in theory) right until the end, however whether the factory continued to stamp the engines accordingly is beyond me.
I believe spitfire (maybe only mk2/3??) engines had the little piled arms stamp near the number, just like the one on the GP
carbs. Though I don’t know if they did this on clubman motors. AIUI it was something that was done to signify motors being assembled in the competition shop. Maybe Gary could confirm?