Master Apron. M.: B.:. - French Rite "Groussier" (RFG)rench Rite Groussier (RFG) Master Mason Apron with Letters M.: B.: Hand Embroidery High End Finishes The Master Mason Apron with the letters M.: B.: for the French Rite Groussier (RFG) is a distinguished emblem of fidelity to the Third Degree and the philosophical depth of the Rite Franais. Designed in accordance with the sober and intellectual character of the Groussier tradition, this apron reflects dignity, symbolism, and refined craftsmanship.