I know the team is trying to keep it under wraps. Word is we will see something on May 10th.
My guess is the 1934 Boston Redskins uniform.
Boston Redskins 1933 uniform artwork (via Scott Sillcox)
I say this for 3 reasons.1) they've never used it before. I think they have done every other throwback version of the uniform.
2) the Steelers 80th throwback would be that same time period.
3) I came across a picture from a skins charity event where the far right guy (not sure the player) appears to be wearing a Boston Redskins hat.
Let me know what you think.