I wasn’t able to find any example photo, but I recently saw some old manuscripts printed in the 18th century (I believe) at the University of Vienna that had mixed letter casing in the middle of the word, e.g. InvIdIa, nostrVM etc. (these examples are not verbatim)
The casing would not emphasize composite nouns or the start of syllables, so I’m wondering if the reason for using them is the lack of enough lowercase casts.
While I do not the exact answer, I do know that solid grammar rules did not exist back when the described manuscripts were printed. Due to this, many improper spellings and cases of odd writing can be seen the further back you go. Also, if it was printed with ink stamps, the printing company may have not had the correct stamps. They weren’t exactly cheap.