T-Bar
Martin's first truss rod
Actually, Martin's first "truss rod" was a simple affair consisting of an ebony piece about 3/16" wide, inserted just below the fingerboard:
In the days of wide 12-fret necks and gut strings, this was sufficient reinforcement.
Once steel strings became standard and narrower 14-fret necks were common, Martin started using a steel insert with a "T" cross section:
This insert made the neck more rigid, and was the standard reinforcement until the mid 1960s, when the 3/8" square tube was introduced:
Some players insist that the Martins with the "T-bar" are better sounding, but that's a difficult thing to prove. Do the guitars sound better because of the neck reinforcement, or simply because they are older? (I think the latter.)