The 911 L was only produced in 1968. The L stood for "Late" (as in late model). The 911 L had the same 130 horsepower as the regular 911. It was also the most expensive of the 911s, since the 911S was not available in the US in 1968.
There were only 449 cars builts with this designation and they all have production dates well after October 1967. This is contributed to engineering & mechanical failures (specifically, the front suspension and transmission). The engineers in Stuttgart, Germany, re-engineered the problem areas and built the remaining 449 cars and subsequently released them for sale.
found here:
http://vista.pca.org/stl/911l.htm