Ebony reinforcement of Martin guitar necks occurred as the guitars were changed over for steel strings. This was replaced by the steel T bar about 1934, beginning with #57305. There was a period during WWII when steel was not available, and guitars of this period would have ebony reinforcement. The T bar was discontinued in favor of the square steel tube in 1967 with #229096
- Martin Guitars / A History by Mike Longworth

and further...

Martin added the T bar when it switched to modern T- shaped frets, partly because it could no longer rely on the wedging action of the solid "bar stock" frets to keep the necks straight under tension.

An invisible change took place inside Martin necks in 1967, when a square steel-reinforcement tube replaced the steel T-bar, which had been used in the necks since 1934. There was still no adjustable truss rod, making Martin on of only two major guitar makers not to use one; the other was the budget-brand Danelectro company. Both companies necks it should be noted have held up remarkably well over the years.
Martin Guitars - Washburn and Johnston


The adjustable truss rod became standard during the latter 80's and around the time of the introduction of the J-40, coinciding with the new "low profile" neck.