The letters apparently stand for Iacobus (Jacobus) Burgundus Molay Bustus Anno Domini Nostra Iesu (jésus) Christi
More info: https://en.wikipedia.org/wiki/Swedish_Rite
1341 was the year Jacques de Molay, the last grand master of the Knights Templar, was burned at the stake https://en.wikipedia.org/wiki/Jacques_de_Molay
This was given to me by an old grand aunt when I was a kid and too young to ask (about 1975), although I am not sure she really knew what it was either. It has taken some sleuthing to find out what it is and what the letters and numbers mean.
This is silver I believe, as the 9th grade would have been gold plated, according to an email shared publicly from Andrew Tucker, Assistant Curator of The Library and Museum of Freemasonry i London (2012)
The pendant is identical back and front.
by LordFondleJoy