One of my favorite books is Bowden's "Napoleon's Grande Armee of 1813" where he "forensically' re-constructs the Grande Armee.
The main sources of French manpower for the corps were the cohorts, depot bns of units (then fleshed out with conscripts) and totally rebuilt veteran regiments. He also mentions the 2 Young Gd regiments (3 Tir, 3 Volt) transferred from Spain.

What Bowden doesn't mention, and what I am speculating, is that there was another source, and that is veteran regiments that served in neither Spain nor Russia in 1812-13. Using Digby Smith's "Napoleon's Regiments" and Tony Broughton's unit lists, I came up with several units that may have been transferred intact (usually from Italy) to Saxony for the campaign: They are the 6th Line, 13th Line, 112th Line, 14th Light, and 22nd Light. If this is true, there was a significant part of the French line regiments that was of higher quality than the rest. It is also plausible that these units were largely "cannibalized" to fill out other regiments.

Can anyone provide any ammunition to support or refute this speculation? What would be great is any regimental histories for these units but I have been unable to locate.