Finding My Great-Great Grandfather

For the last 15 or so years, I have been trying to find out who my great-grandfather's (Clarence Hough) biological father was. This has proven extremely difficult for the following reasons:

1. He was born to unmarried, unattached Ella Hough in 1909 in Oklahoma, where birth certificates were not yet commonplace (and his probably wouldn't have had his father's name, anyway).

2. He was given his mother's surname, and subsequently adopted by her great-uncle who had the same surname, so every available record lists him as the father.

3. As far as I can gather from living relatives, Ella never told anyone who the father was.

I've chased down multiple theories over the years, but none have panned out. My own DNA matches haven't helped much, so this Christmas, I convinced my dad (Clarence's oldest living descendant) to take a test. I just got his results back, and the results are...somewhat promising? I'm hoping some of you fine folks can help me best make use of them.

The vast majority of his results, I recognize as being from lines I'm already aware of, but there is one group of matches that are all related to one another but don't have any clear connection to us. I'm hopeful that this means they are all descended from my mystery great-great-grandfather's line, but I'm having trouble figuring out the best way to approach this. I have messaged several of those matches, and looked through any public trees they have, but nothing has jumped out so far.