George Marker - George Marker Middle Name

I am trying to determine my Great Grandfather, George Marker middle name in order to follow the ancestry into further generations up
My father was John Michael Marker, his father was John William Marker- John William died when my father was 10 years old.
I have seen George W and George W I cannot determine which is correct
I am trying to find documentation to make sure I am following the correct path. Any help would be appreciated.
Thank you
Margaret Marker Hopkins