Probably San Augustine, but my understanding is the original Co. C became Co. K after the 4th Battalion was consolidated to become the 27th Cavalry aka Whitfield's Legion. Bill Nolan is the authority and he will know for certain. One of my ancestors served in Co. K and he enlisted in San Augustine in 1861.

http://www.rosstexascavalrybrigade.com/27thtexascavalryregiment/27tcrc.mhtml