Update: Got the key out but discovered that the tang was an integral part of the barrel assembly. Did manage to get the two screws broken lose and out and then was able to remove the barrels. Here's what I found.
Here's a closeup of one of the four identical proof marks.
This is a sketch of what looks to be inside the proof mark under the crown.
Six inches up the right barrel from the breech are the letters W M
(W Morse ??)
One inch from the breech of the left barrel are the letters W A
Note: Both barrels have either one one or eleven stamped on them and the left barrel has two sets of hash marks, one set lighter than the other.
Jon and Miller and Drew: Thanks so much for your interest in this family mystery. I would appreciate your (as well as others') insight regarding this gun and particularly regarding the question: "Is it of the period that it could have come west in the late 1860's (or perhaps early 1870's)?
