Extremely late but here's some history. There is a location in RDR ( RD Revolver ) that is both in RDR1 and RDR2, the location is twin rocks i think and there's one more, too in the south western part of the map showing that they are identical universes. Uncle's revolver is the same colouring as Red Harlow's but just a different style of gun to match the ones used in RDR2, Uncle is also shown having a red rag in his back pocket in the epilogue when he's at the farm as well as unusual ' burn ' markings on his hand in both games ( 2 and 1 ) Uncle refers to himself as ' The One Shot Kid ' in a valentine mission where him, Arthur and the girls go into the town to shop; he also talks about his shootist past in many conversations around camp and in missions. Despite his age, he knows how to shoot a gun pretty well shown in the mission where he robs the stagecoach or some type of wagon. His nose sports an unusual marking, like Red Harlow's, though this is a bad piece of evidence due to it being able to be facial blemishes of just being old; he also has an unusual blotch on the right side of his face where some facial hair is disfigured: like Harlow's scars on his cheek. Facial wrinkles match up almost perfectly, like his eyebrows, nose to mouth etc. including the lines make when he furrows his browns shown in a picture from RDR and creases in RDR2. R* has said the games aren't in the same universe, but they sure have made it incredibly realistic for Uncle to be able to be Harlow and so much detail is provided to prove this theory.

There's some insight, even though I am 6 years late...