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.