Why do you believe Kluk z plákátu is the original title?

Title: O klukovi** z **plakátu I mean Z O klukovi s plakátu its wrong

"O klukovi s plakátu" was the original series title used in the Season 1 (1970) and Season 2 (1974). It was grammatically required to use "s" preposition back then, but the grammar changed in 1974 and the "z" preposition started to be used. Therefore, the Season 3 made in 1982 used the new title "O klukovi z plakátu" as well as all subsequent series rebroadcasts (original Season 1 a 2 on-screen titles didn't change, though).

As for the original movie and series name here on TMDb, we have a rule to use original text as written in the original on-screen opening credits. However, there is an exception regarding the series name change during its initial broadcast, which could be applied in this case (if I understand the rule correctly). So if @banana_girl is fine with it, I would change the series name as @Drobekian suggested and add the old title as an alternative.

Thanks for the incredibly enlightening explanation @talestalker.

However, there is an exception regarding the series name change during its initial broadcast, which could be applied in this case (if I understand the rule correctly).

Yes, it think the exception does apply here.