Bit hard to nail down an exact date. The Columbus tubing that was first drawn in 1930 for bicycle frame building was probably close to the same alloy and dimensions as what would later be called "SL", but from 1972 thru about 1977 at the earliest, there was no SL on the tubing decals, just gold foil and later (about 1978 thru 1984) the blue bordered decal which
also would be used on SP tubing frames.
It was probably noted as SL in Columbus catalogs, and so spec'd as such by builders, but I can't find the earliest mention of that in print.
Magazine ad in 1975 mentions the 5 tubing sets that were in the market that year (still using just a gold foil decal for all 5
AFAIK): SL, SP, PL, PS and Rekord.
Pic of the earliest gold foil decal that Robt. Broderick says was in use from 1972 to 1974, the next 2 iterations look about the same except the dove eventually got printed in white ink.