Thayer David appeared in two episodes of WWW--"The Night of the Samurai," which aired on Oct. 13, 1967 (and thus, a little over a month before he returned to DS in the role of Ben Stokes), in which he played a sinister Chinese (or was it a Russian who liked to dress Chinese? I can't recall). And "The Night of the Spanish Curse," in which he played the crazy guy impersonating the spirit of a dead Conquistador. The latter aired on Jan. 10, 1969, but must have been filmed at some point in late 1968, I would think.
Thayer David was a very busy actor. He may well have worked himself to death. I did finally hear from Thayer's nephew that his passing was sudden and quite peaceful. He sat down in a big chair to have a nap and never woke up.
Best, G.