In his later years
From 1964 to 1968, Del Monroe played Seaman Kowalski in the Irwin Allen TV series 'Voyage To The Bottom Of The Sea'. (In the Cineverse, his character was known as Kowski.) No first name was given for Kowalski at any point in nearly 200 episodes.
Nearly a decade after the show ended, Del Monroe appeared in an episode of 'The Rockford Files' - "Where's Houston?"* In this he was known as Charlie, no last name given. Charlie was the main enforcer doing the dirty work for a corrupt developer named Charles Blackhorn.
So Charlie had no last name and Kowalski had no first name. But I can't make the claim that they were one and the same for two reasons. 'Voyage To The Bottom Of The Sea' fans would rise up against me for besmirching the good name of Seaman Kowalski by saying that he "turned to the dark side" after leaving the Nelson Institute for Marine Research.
I also can't claim that Charlie was Kowalski's evil twin brother for the second reason I can't conflate the two: the characters as we know them appeared at almost the same point in the Toobworld timeline, even though nine years had passed between 'Voyage To The Bottom Of The Sea' and that episode of 'The Rockford Files'.
'Voyage To The Bottom Of The Sea' may have been produced between 1964 and 1968, but the pilot episode, "Eleven Days To Zero", takes place in 1972 (in the original timeline for Earth Prime-Time as a matter of fact - before Professor Nick Cutter inadvertantly altered it back in prehistoric times during one of his trips through a 'Primeval' timeline.)
And Jim Rockford was threatened by Charlie in 1977, just four years later when Seaman Kowalski was still serving on board the Seaview under Admiral Harriman Nelson and Captain Lee Crane.
This is how Seaman Kowalski appeared (basically) throughout his tour of duty, at least for the time we saw:
And this is how Charlie looked in "Where's Houston?" (seen right after his underlings gave Rockford a beating):
I have no problems in accepting Charlie as a Kowalski though. He would have been the older brother to the Seaview seaman and just a nearly identical brother... but not a twin.
* Monroe actually appeared in a second episode - "This Case Is Closed". But he played a guy in New Jersey named Vic......