I can't remember from where I know this, but my memory says that Gibson had a patent on adjustable truss rods which expired about 1940, preventing other firms from using them. Many pre-WWII steel-strung guitars had a reinforcing strip up the centre of the neck. My uncertain memory also says that Martins had a T-section reinforcing bar from about 1931 - the square bar came later, and they did not use an adjustable bar until about the mid-60s.