If you look at my
post #31, the Guild employee who built
@DThomasC one could have potentially built the OP's one even earlier. The neck serial number 140020 is something like mid 1976, a bit earlier than the bolt-on neck S-300s. And the shape of the routed chamber was also odd on DThomasC one as you see below. The neck has 24 frets already, maybe they were thinking to have the necks colored first and that was a leftover neck that was stamped but never used due to the black color.
If that shape of the body was something Guild was thinking about is unknown to me, only Hans would probably know. But it is not fitting a lot of things which are typical to Guild, so the shape is something very different.
In any case I'd say that guitar was not built as official Guild prototype, I would think it is similar to the story of DThomasC one, finished outside of the Guild factory most probably by a Guild employee with access to the correct parts. Might have even used the Guild spray booth for the black color. The harness is also original Guild but has a 1974 date code, so the pots are early.
Here's the full picture of the odd shaped routing in DThomasC one:
Vs the OP's one, somehow, many of the same features in the odd routing:
Ralf