The gun you write about was on public display about three years ago when I visited the Palacio. It is a nice rifle, but something in the corner of my mind says that there were some oddities about it. Don't recall now, maybe refinished, barrel cut back a bit, and I may be confusing it with the one up in Taos. Too, long ago for an old man to get it 100% right.
Barring the chance to go to Santa Fe and get the information you want first hand, phone the curator of the museum in the Palace to start the process of getting what you need. I am sure that they would be highly gratified to talk with you as you will no doubt have family information that they would want; diaries, genealogies, old photographs, family held items, and so on. Good luck, yours is a noble cause.
Dick