I’ve read in one “tech fandom” booklet, From the Files of Star Fleet Command (1980), the idea that smooth-headed Klingons are from the planet Kazh and bumpy-headed Klingons are from the planet Kahless. It suggests that control of the Empire had passed back and forth between the two species a couple of times, i.e., obviously Kazh was dominant during the TOS era, and Kahless by the time of the TMP.
The name Kazh comes from another fan source, The Best of Trek (1978).
I’m fine with the various other explanations (Ford’s Imperial Race and Human-Fusion Klingons, or what ENT went with), but honestly the Kazh/Kahless explanation is the cleanest, simplest, and most flavorful, to me.

The name Kazh comes from another fan source, The Best of Trek (1978).
I’m fine with the various other explanations (Ford’s Imperial Race and Human-Fusion Klingons, or what ENT went with), but honestly the Kazh/Kahless explanation is the cleanest, simplest, and most flavorful, to me.