15L refers to the crank advance degrees. it actually doubles, so 15 = 30 degrees crank advance. On my car, the factory initial timing was set at 6 degrees and the distributor limit slot was a 15L (or L15) The equated to a total of 36 degrees total mechanical timing without added vacuum advance.
Here is a pic of what I mean by the 15L limit, but actually, this one is set at 13L or 26 degrees crank advance. In this instance, 26 + 6 = 32 total mechanical which would be ok to set the initial to 10 degrees to equal 36 degrees total. One 'trick' that could be done as seen, would be to add a sleeve of Nylon to the post. This would in effect cut the limited advance and allow possibly 12-14 degrees of initial timing without trying to find, or in my case weld up and recut the slot to equal a 10L slot. On top of all this, the springs used and the tension they are set at make for how the "curve works". It gets to be fun for sure.
Hope that explains it a bit better.