We did not adopt the L prefix. They were modified beginning around 1967 and then became the C-1 GPMG. They remained with that designation until the about 1979 when defects were corrected with a major upgrade and they were redisignated C5A1.
I have the some of the modification instructions at home for both of these changes, so could verify those dates if need be, but they will be pretty close.
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