Porsche-Excellence Was Expected lists "917L" for the long-tails, "917LH" does not appear.
Here is my take on it. The original 917 was sold with two tail sections; long-tail and short-tail.
Original long-tail:
Original short-tail:
I believe the "917LH" designation was adopted for the later 1970/1971 long-tail versions to differentiate them from the original 1969 iteration.