Martins always did have neck reinforcement. In the pre adjustable truss rod era, they had a length of 3/8" square steel box section embedded in the neck under the fretboard. (Much) older models used to have strips of ebony inset in the neck.

I always take it for granted that when we talk about "truss rods" we are by default talking about adjustable rods. An inert embedded bar I would refer to as a "reinforcing bar" which mechanically is a slightly different beast.

Murray