The lion's share (if not all) of badges made after 1936 have this mark.
"本 inside the oval".
But what 本 stands for?
Original wooden case for the membership badge which will help answer this question.
本部製 - Made by Headquarters
So 本 stands for 本部 - Headquarters of Military...
japanese badge marked with 本
japanese military reservist association badge
mark 本 inside the oval
marked japanese reservist badge
reservist association membership badge
帝国在郷軍人会章
帝国在郷軍人会章本部製
本部製