Nice. You're right about what it is. It's from the Republic of Texas (pre Civil War), listed as Albert's SU 387 and is said to be very rare. Great find!
(pardon my duplication of this post from the thread about this item in Today's Finds)
Albert's classification system is considered the standard ID system (although I welcome correction on this if it is no longer the case). I use the encyclopedic book "Uniform Buttons Of The United States 1776-1865" by Warren Tice, which references Albert's numbers, but includes his own numbering system. Your button in this book is TX226A1.
What an awesome find for Texana buffs! It has to be extremely rare as The Republic of Texas only existed from March 2, 1836, to February 19, 1846 before it became part of the US.