So the 6 ply Ludwig was similar to the Gretsch except they used poplar for the 4 inner plies instead the the 4 plies of Gumwood that Gretsch (Jasper) was using. The Ludwig shell was thicker than the Gretsch too.From what I've been told, the original Classic shell was two "3-ply-like" layups, one inside the other.
When the shell finally went into production, the layup was changed to 4 middle plies of Poplar surrounded by 2 Maple plies.
The 4 ply shell just had two Poplar plies removed.
It wasn't until the 5 ply shell that those Poplar middle plies became thinner (probably veneer availability combined with more modern slicing techniques).
Both Ludwig and Slingerland dropped the 3 ply shells with the single scarf joint for shells like Gretsch had been using for a couple of decades with the staggered joints. I don't recall which of the two Chicago giants made that move first (I think Slingerland's 5 ply beat Ludwig's 6) but once one of them made the change it was inevitable that the other would have to follow.