OSP says
Libertinage is a remix of Linux Libertine and was designed in August 2008.
For more detailed information: ospublish.constantvzw.org
We built Libertinage by copying and pasting parts of Linux Libertine glyphs. There are 26 variations, one for each latin letter in the alphabet.
Libertinage.ttf is the 'Full' version, containing all modifications.
Single letter versions are named Libertinage-a, Libertinage-b, Libertinage-c... depending on the letter that was changed. All 26 are gathered in the Libertinage package as .ttf files.