Very cool find. Note that the initials on the back are actually ‘RADJ’ rather than ‘RAD’, with the ‘J’ standing for ‘jugend’ (youth). When the ReichsArbeitsDienst was formed on 11 July 1934 by renaming of the former NationalSozialistischer ArbeitDienst (NSAD), there were two major sections. One for men: (Reichsarbeitsdienst Männer – RAD/M); and one for young women (weiblichen Jugend – RAD/wJ). The latter was initially voluntary, and then compulsory from 1939 onwards.
I don’t know if there were separate insignia pins for boys in the Hitler Youth movement, but my understanding is that when the ‘RAD’ is appended with either ‘wJ’ or just ‘J’, it relates to the female youth section weiblichen Jugend.
There were several makers of these pins, and the M.O.S. initials are for Matthias Oeschsler & Sohn of Ansbach in Germany.