The abbreviation "p.m." stands for "past master." "A.F. & A.M." is the abbreviation for "Ancient, Free and Accepted Masons."
Lewis was elected to membership in the brotherhood of Freemasons on January 28, 1797, and rose quickly through the consecutive Degrees until he attained the status of Past Master Mason in early April. Two years later he advanced to the level of Royal Arch Mason in Widow's Son Lodge, Milton, Virginia, a village situated about thirty miles down the James River from Richmond.
Three years after the expedition, on September 18, 1809, William Clark was admitted to St. Louis Lodge No. 111, A.F. & A.M., which Lewis had helped establish a year earlier.
As Stephen Ambrose has pointed out, Lewis took the ritual and idealism of the Masons quite seriously, even naming three tributaries of Jefferson's River for three virtues celebrated in Masonic ritual—Philosophy, Wisdom, and Philanthropy.
Eldon G. Chuinard, Lewis and Clark: Master Masons, We Proceeded On, Vol. 15, No. 1 (February, 1989).