I can't remember where I know this from, but I seem to remember that Gibson had a patent on adjustable truss rods which expired around 1940. Non-adjustable rods were fitted to some Martins pre-WWII - I think this started about 1931, and the rods were T-section - the square rods were later. 30s guitars which had steel rods were definitely made by Larsen bros. (Maurer, Prairie State, Euphenon) - the rods on these go right down through the soundbox to the endblock. Many pre-WWII steel-string guitars have an ebony or maple strip up the centre of the neck. I do not know if this was a feature of Martins fitted with the T section rod. Anybody volunteering to saw their guitar neck in half to find out?