And that’s why

 I’ve noticed a weird typographical mark on old post boxes:

P indicates “post”and T “telegraph,” so obviously the mark in the middle means “and” —but why? 

Today on reddit I learned that Roman politician Cicero employed a secretary, Tiro, who invented a form of shorthand that became known as Tironian. The ⁊ was his symbol for “et.” When the Latin writing system arrived in Ireland 450 years later, so did Tironian shorthand. Over the next 1500 years, Irish writers preserved the mark, which they would call “agus,” in Irish writing system.

According to wikipedia, on the continent, Tironian fell out of favor with the rise of witchcraft, to which it was associated.

In contrast to the corners of Latin letterforms, Tironian does look colloquial and mysterious.