Just as in English we might use a spelling such as “5th” for “fifth,” Spanish speakers often abbreviate ordinal numbers using the numerals themselves. A big difference in Spanish is that the abbreviations vary with gender.
For example, octavo (eighth) is written as 8o if it’s masculine and 8a if it’s feminine. Such forms aren’t common for numbers above 10. Note that in masculine forms a superscripted zero is used rather than a degree symbol.
Have you seen these words before? Have you had trouble understanding them?