This is true. I have a '82 D150 and a '84 D100.

The difference is GVW rating.
In the mid '70s, catalytic convertors were required on trucks if they were under a certain GVW. Bumping the GVW up on a D100 to a new D150 model exempted the need for a CC.
Eventually the CC requirement GVW was raised over the years. The last year the D150 was exempt was 1978, '79 150s had CCs. The D250 and 350 exemption went into the '80s. I had a '84 D350 that came without CCs.