Things from my uncertain memory:

1) Gibson had a patent on adjustable truss rods which expired about 1940, preventing other makers from using them.

2) Martin used a T-section reinforcing bar from about 1931 onwards

3) Many pre-WWII steel-string guitars were reinforced with an ebony or maple strip in the centre of the neck

4) The square bar in Martins came post-WWII, and was used up into the 60s at least.