Okay, that might have kicked my Red Dead fandom into overdrive. I think that was maybe a Rockstar employee’s personal map. I cleaned it up and added the RDR map. Some of the text was really hard to decipher, so I could be wrong!
I obtained a screenshot of the different landmass chunks in Red Dead from an extracted archive on the XBOX disc. Using that I saw it fits exactly with the same square chunks on the leaked image, which makes me think the maps’s real. I made an updated version of the map which includes the added RDR terrain. You can see the extracted streaming chunks from Red Dead Redemption matches up perfectly with the cross over terrain. I also included a black & white version for readability.
