I seems to me that Slate's resignation was premature. Perhaps he felt his King was too far away. However, I can't see how Black can make progress: His Rook is tied to his c-Pawn and moving his King over to support it would allow White to get his King over to a2 with an almost certain draw (assuming best play, which might be a stretch for mere mortals playing a R+P endgame). By the way, the notoriety of the R+RP+BP vs R endgame goes back to Marshall-Rubinstein, San Sebastian, 1911 and it is just as drawn on a Gothic board as it is on a regular chess board since the dynamics of R+P endings are determined by how close the closer side of the board is to the Pawn(s); the extra width on the other side makes no difference (as long as the Pawns are all on one side of the board).