According to this site, upon reaching their destination (VA) the troops were formally organized into regimental companies bearing numbers and letters representing the order of their departure from Camp Van Dorn.

You would think Henry Travis should have known his new unit# prior to his letter dated 11/22/1861 from "Camp Hood".

http://www.pha.jhu.edu/~dag/4thtex/history/history.html#oct61

I still don't have a clue regarding G. F. Foster, Procter P. Porter was serving as the captain of Company H / 4th. TX Inf. when this letter was written.