No text, but a visual code

[davidjackson]

Smith in History of Mathematics vol II (pp234-235) says the º form starts to appear in the mid 1500's, he gives two examples, a printed work of Schoner, De logistica exagenaria, 1569 on p..366, which speaks of "qui & partium numerus dicitur, circulus, 7 unitates". There the º is written above the number. 
And Peletier in 1549 in L'Arithmetique p.107 (1607 ed)

Peletier remarks "Les Degrés dont seront au milieu de la numeration Astronomique: & seront represetés par º : les Minutes par Ï les Secondes par ¨2... Et ansi des autres".

So it appears the º was a printers convention that came into use in the middle of the 16th century.

Smith gives earlier manuscript examples of ~, ¨ or '' for degrees. But he is clear that: 

The symbols (º ' '') are modern. In medieval and Renaissance times there were several methods used for designating the sexagesimal orders [...] At the close of the 16th century there came into use such forms as these in which the symbols are obviously the forerunners of the ones now seen in common use.