Mattithiah ... the first-born of Shallum the Korahite - This Shallum would seem to be the person mentioned in 1 Chronicles 9:19, whose actual first-born was Zechariah 1 Chronicles 26:2. Mattithiah may have been his eldest lineal descendant at the time here spoken of.
Related Commentaries of 1 Chronicles 9:31
1 Chronicles 9:31
31 And Mattithiah, one of the Levites, who was the firstborn of Shallum the Korahite, had the set office over the things that were made in the pans.