There's the H for Henri II, intertwined above with D for his mistress, Diane de Poitiers. Some say there's also a C for his wife, Catherine de Medici, but I'm guessing that's just rationalizing.
You've got K for Charles IX, the second son of Henri II.
L intertwined with A representing Louis XIII and his wife Anne of Austria who by the way was Spanish, not Austrian.
A generic L sometimes with a B for Bourbon, perhaps representing Louis XIV, XV, and XVI.
NL for Louis-Napoleon, not the short guy from Corsica, but his nephew who became Napoleon III, leader of the Second Empire.
Not all of these initials are authentic to the time period of the kings they represent. Many were destroyed during the Revolution and in the subsequent upheavals during the 19th century. If you want to learn more, read this interesting article on the Louvre's Web site. And next time round, go on your own treasure hunt for remnants of French history.