In a 16th century Latin treatise published in Poland several occurences of the final m have the form of ꝫ (in Unicode U+A76B LATIN SMALL LETTER ET). You can find more information about the work at https://github.com/jsbien/Zaborowski-index4djview. An illustration demonstrating that sonum and scilitet end with the same letter can be found in the preprint https://www.researchgate.net/publication/341930612 (available on request). Is there an explanation? I will appreciate your comments.
According to the introduction to Capelli's abbreviation dictionary (section 4.281), as translated by Heimann and Kay:
When the ꝫ-mark occurs at the end of a word and is preceded by the vowel a, e, or u, it generally does not stand for -us or -et, but rather for m. It is almost always written on the same line as the other letters in the word, for example: naꝫ = nam; o̅e̅ꝫ = omnem; h̅i̅tuꝫ = habitum.
(Section 4.28 in general is dedicated to this sign, and goes through various other meanings in other contexts, such as -nem with preceded by o or -rum when marked with an additional slash.)