Your grandpa maybe thought Mosty belonged to Poland because..
Mosty u Jablunkova- ..the censuses conducted in 1880–1910 the population of the municipality grew from 1,959 in 1880 to 2,318 in 1910 with a majority being native Polish-speakers (growing from 98% in 1880 to 98.5% in 1910) accompanied by German-speaking (at most 2% in 1880) and Czech-speaking people (at most 0.3% in 1900). You could probably assume that a majority were probably culturally Polish as well.
Piątkowski, Kazimierz (1918). Stosunki narodowościowe w Księstwie Cieszyńskiem (in Polish). Cieszyn: Macierz Szkolna Księstwa Cieszyńskiego. pp. 267, 285. (Yeah, I cheated- Wikipedia. How is your Polish?)
Your grandpa maybe thought Mosty belonged to Poland because..
Mosty u Jablunkova- ..the censuses conducted in 1880–1910 the population of the municipality grew from 1,959 in 1880 to 2,318 in 1910 with a majority being native Polish-speakers (growing from 98% in 1880 to 98.5% in 1910) accompanied by German-speaking (at most 2% in 1880) and Czech-speaking people (at most 0.3% in 1900). You could probably assume that a majority were probably culturally Polish as well.
Piątkowski, Kazimierz (1918). Stosunki narodowościowe w Księstwie Cieszyńskiem (in Polish). Cieszyn: Macierz Szkolna Księstwa Cieszyńskiego. pp. 267, 285. (Yeah, I cheated- Wikipedia. How is your Polish?)