Never noticed that.
Usually on 60ies Guilds there was a sheet of maple plywod (?) or some similar material under it.
Example of a removed 60ies overlay and you still see remaining pieces of the "maple barrier layer" on it:
Here you see the new overlays with the attached "barrier layer" on the back (I marked them). So they came to the factory as a two layer sandwhich, one layer black celluloid and below the glued on barrier layer. That also helped speeding up to glue the veneer easier on the headstock as you now only had a wood to wood connection instead a celluloid to wood connection I think.
On those shrunk overlays you can see the sandwhich construction. First the very thin black celluloid which in that case was peeling away the roughly 1mm thick maple (or whatever material it is), in that case still with the red color on it. The bare wood visible is the mahogany headstock.
In the below example only the black celluloid shrunk and the maple (or whatever light material) layer is still firmly attached to the mahogany headstock:
So it was news to me seeing guitars where the black veneer was glued directly on the headstock without that layer in between.
So I only can guess that this happened at a time when the factory stopped using black celluloid veneers and replaced them with other black material for the headstock veneers. Somehow the in between layer got missing...
Ralf