http://cokin.com/en/149-creative-range
Might take a bit of trawling through but they're all listed on their site.
Have you waded through their website?
I'd guess that L and M relate to the filter size. Looking at their website there seem to be two types now, creative and nuances, but maybe in the past there were more or others.
I have Cokin filters and from what I remember something, either filters or holders or something, came with a little book with lots of stuff in it... I don't know where it is now though.
L refers to Light 1 stop
M medium 2 stop
and no letter 3 stop.
the number is for the type of filter 121 i think is soft grad ND from memory