On ebay, they're interchangeable.
But in reality, L39 = LTM = LSM, leica thread mount or Leica screw mount, with a flange-distance of 28.8mm, and 26 Threads-per-inch. Mostly it should include range-finder coupling but sometimes don't (like on wide-angles).
M39 generally *should* only apply to the Zenit SLR, flange-distance of 45.2mm and 1mm thread-pitch. It can also refer to 0.75mm thread-pitch, as used on filters. Or as you can read on the internet, it also refers to LTM even though it shouldn't.