I often see "L" for Labour and "K" for Capital, as well as "M" for natural resources (i.e. Land) but they don't seem to be applied consistently.
In economic models, K is commonly used to represent "capital." This is presumably due to the fact that German for capital is "kapital," and also to the fact the C is more commonly used to represent consumption.
There is no difference in meaning between labor and labour. Labor is the preferred spelling in American English, and labour is preferred throughout the rest of the English-speaking world.
I believe it really depends what country the economic authors or book writers are from.