[The writers for Yes Minister acknowledged that this wasn't their invention]
The "tits" punchline used in later versions of the joke makes more sense in an era where the association between the role of Prime Minister and male gender is shaken, which in 1973 may still have seemed like a distant future possibility - whereas by the time Yes Minister was first broadcast Margaret Thatcher was in 10 Downing Street.
https://www.dirtyfeed.org/2021/04/what-the-papers-say/
[The writers for Yes Minister acknowledged that this wasn't their invention]
The "tits" punchline used in later versions of the joke makes more sense in an era where the association between the role of Prime Minister and male gender is shaken, which in 1973 may still have seemed like a distant future possibility - whereas by the time Yes Minister was first broadcast Margaret Thatcher was in 10 Downing Street.