For what it's worth, overlapping letters, creating ligatures, and inserting dashes into/over letters is quite common as a feature of shorthand for many commonly used words or letter clusters in pre-print writing systems. In the Latin-based system, for example, common words might be shortened to just the beginning and ending letters with a line over them; the word "and" (Latin et) was shortened into the ampersand, which doesn't necessitate starting a new letter; and common grammatical endings are often contracted with a dash or other mark to save the scribe some work. With typesetting, this style remains both because it is familiar but also because commonly used symbols can get a symbol (or key on the modern keyboard) which again saves a bit of time for the typesetter or typist. The overlapping of letters or using dashes or other marks was a familiar technique, rather than one invented for the purpose of designating currencies.
I can speak on Roman currency and explain the British symbol for the pound. With the Roman numerical system, the letter "X" could mean:
To differentiate between all of that, they'd put a strike through the letters to indicate it's a number. So for example the sestertius had a value (at one time) of 2.5 as and would be written as IIS with a strikethrough. Early printers would replicate that by using "HS" which was the closest thing they could get.
Now when the English decided to adopt a new coinage system, they modeled it on the Roman system to hearken back to the stability of the Roman denarii (and somewhere Classicists are snickering) so you end up with that bizarre fraction system. They used the Latin word for pound (libra) for the higher denomination and to indicate it represented a number (you can see where this is going) they put a strikethrough the letter "L".
As for the rest of the world, I have no idea.