How did "combo" arise as an abbreviation for "combination"?

I am interested in the shortening of "combination" to "combo." I understand the etymology of "combination," and it’s clear to me how "comb-" would be used taken for the shorthand.

I see several sources indicate that it was first used with jazz/musical groups in the 1920s (Wiktionary, Etymonline).

Where does the "-o" ending come from? Is there a common convention or parallel word that the -o ending could have arisen from? Or is it something arbitrary that caught on?