^I've always taken "regular" to mean someone who appears in, or at least is credited in the main cast list of, every episode. The supporting TOS players were never more than semi-regulars. Even Kelley was missing from two first-season episodes. Nichols was in 65 episodes, Doohan in 64, but Takei in only 51 episodes and Koenig in only 35.
Well, even in the 60's there were leads, regulars, semi-regulars and recurring. Ultimately, I donít think it had as much to do with the credits since that was contractually based and not necessarily how the show developed. Let's take the Irwin Allen shows as an example: Lost in Space and Land of the Giants listed all of their main cast up front in the opening credits. Yet, some of the characters in both shows were marginalized down to semi-regular status. In the case of Lost in Space, the lead was practically a highly paid a day player once Dr. Smith took over.
However, Voyage to the Bottom of the Sea and The Time Tunnel only listed the two leads. The end credits listed EVERYONE, even the leads. You can still be a "regular" if you missed a couple of episodes. On Voyage, Bob Dowdell (Chip Morton) was indeed a regular, but he missed at least two episodes during the run. His credit at the end was also in a larger font, preceded by an "and." Starting with the 2nd season, Terry Becker joined the cast as Chief Sharkey and he was in every episode (except for a batch because of a contract dispute got him fired temporarily). Neither he nor Dowdell were semi-regulars, they were regulars. Arguably, Becker should have been moved to the opening credits, he was as important to the show as Kelley was on Trek. As for the three "end credit" cast members on The Time Tunnel who manned the controls, they were in every episode, but they weren't the leads. They were regulars.
I see why you might use the credits as a guide, but it's not consistent across the dial. On Trek, the guys up front were the leads, with the back end guys being regulars, semi-regulars and recurring depending on their contributions to the series.