There is a minhag to say at the end of Shemoneh Esrei and Kaddish during the 10 days of teshuva, "oseh hashalom" instead of "oseh shalom". This is mentioned in the Mateh Ephraim in Siman 582 as well as the Kitzur Shulchan Aruch in Siman 192. Is there any other place where this minhag is brought down? Does the Mishneh Brurah bring this at all? What's the earliest makor (source) for this minhag (custom)?
The Sefer Ishei Yisroel brings the Mateh Epharaim(like you sourced) and brought the Likutei Maharich chelek 3 hilchos Rosh Hashanah 65 who brings a reason ,and also the Eretz Yehudah is another source brought.