Bob, it's a might big longshot that you will ever come up with a serial number of his Spencer or a number on his pistol. I can't believe that the Battle of Beecher's Island has never been made into a movie, it has it all Indians, cavalry, scouts and frontiersmen fighting for about 9 days from a sandbar of a small river, with indians on the hills penning down the the small army of men at the sandbar, until a scout gets through and brings the CAVALRY. again saved by the Cavalry!
Now back to your question, there is a museum near the battlefield at Wray, Colorado. I would start there and see if they might have any information on the Spencers used at the battle by the enlisted scouts. Good luck