No text, but a visual code

[davidjackson]

Well, a º is a very easy and quick symbol to draw. 
No we don't know why º was chosen as a degree sign, but I suspect it is simply a printer's decision, as it would be a symbol he already had in his typeset and which would be difficult to confuse, same for ' and '' . (º as a superscript suffix was in common usage when denoting ordinal numbers in manuscripts, as most ordinals in Latin end in o anyway).