I'm sure you already know this, but,,, if it can be absolutely traced to the Alamo, the price will be considerable!
With just some initials on it, the value will be that of a Soddy rifle, considering condition.
I've only seen a few Soddy rifles for sale lately, and considering condition, seem to range up to about $4000.
One question, as I'm not that well versed on Soddy guns; were they being made prior to 1836, in a recognizable Soddy configuration?
Any pictures of the gun?