I didn't know much of anything about this when I purchased it; I just really fell in love with that carved hand gripping balanced scale, which speaks of justice fairly delivered to me! Certainly it appeared to me to be Masonic, and with a bit research I've learned that it is a keystone indicative of the Royal Arch Masonry (also known as "Capitular Masonry")--the first part of the American York Rite system of Masonic degrees and said by many to be "the most beautiful degree" in all of Freemasonry. The letters, which read clockwise, starting at 12 o'clock, stand for "Hiram, Tyrian, Widow's Son, Sendeth to King Solomon." There is a very complex (and quite cryptic to the uninitiated!) set of meanings at play here, and corresponding rituals, but from what I understand the keystone is central to them, and part of the achievement of the "Mark Master Mason" degree involves the creation of a keystone, with that mason placing his own mark at the center of the letters.
I've only found a couple of example of full scale carved stone keystones of a related ilk to this one, but no others with a scale or as beautifully done. Whether this was actually used as an architectural keystone I'm not certain, but it seems it could have been. A wonderful sculptural object now--and it seems an icon of balanced judgement is always a good thing.
7 3/16" t x 5 1/16" w x 1 3/4" d; weighs just over 5 1/2 pounds. Very good condition, with a great patina. Later 19th c. I believe.