Fast forward to about six weeks ago. Some of the birth records on the Skrudzina microfilm have been indexed and are now searchable on FamilySearch. I find two Krupa families in Skrudzina that are having children around the time Sophia is born:
1.) Joseph Krupa and Catharina Czyrpak, House #16: Children born 1883, 1884, 1885 (none named Sophia)
2.) Paul Krupa and Sophia Hejmej, House #51: Children born 1886, 1890, 1892 (all boys)
So, there IS a Joseph and a Catharine there; Catharine's maiden name is different than what I was looking for, but no baby in 1888. Apart from these families, there is one more 'Krupa' birth record from Skrudzina that I find in the index:
I did a quick search of Jacob Krupa and Mariae Kotodziej, and found six indexed baptism records for their children, ranging from 1843 to 1865. However, none indicated the name of Catharina or Paul. The family lived in Obidza, Poland, which today is just a 20 minute drive from Skrudzina. These records are on a different microfilm, so I have a feeling I'll be ordering that one, too.
Did Sophia live her life believing that her parents' true names were those she listed on the Social Security application? Perhaps she was raised by the Joseph and Catharine Krupa in House #16 and she genuinely thought that they were her biological parents. Or maybe she knew the complete truth. That's probably something I'll never find out, but hopefully ordering these microfilms will help me learn more about her mother and the other families in Skrudzina and Obidza.