Before LMT started laser engraving their standard upper receivers in the 2000s were they marked with an L much like the Colts with a C? It seems some prototype 6.8 weapons used LMT receivers.

Per LMT directly;

Regular M4 style uppers have never had any special markings beyond our LMT laser mark.  Before we started laser marking them "LMT" on the opposite side of the ejection port, there were no distinguishing markings on those uppers at all - we've never marked our uppers with a "L" like Colt did with a "C".