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.
Excellent point. There was not such a hardline definition of series "regular" at the time that an actor not listed in the main title was instantly bumped down to a "semi regular" or anything less, particularly if one considers his contributions to the series as a functioning character.
One could even argue Lew Parker's That Girl
character Lew Marie (who appeared in 65 of all 136 episodes) was a "regular," as his character was not simply recurring, but a functioning part of the developing plot of the series despite never being listed in the main credits.