Crusty Curmudgeon
This road is most definitely FM 1188. This section of FM 1715 was transferred to FM 1189 in 1984. 

Source: Transportation Planning and Programming Division (n.d.). "Farm to Market Road No. 1188". Highway Designation Files. Texas Department of Transportation.
URL: https://www.dot.state.tx.us/tpp/hwy/FM1000/FM1188.htm 