The International Correspondence Chess Federation (ICCF) uses a concise, unambiguous format for recording moves, using only the numbers 1 to 8.
Castling is indicated as an ordinary move by the king. The rest of the castling process (the rook's movement) is not explicitly indicated.
A move of 2133 would mean that the piece in the indicated square (a knight in this case) moves one file to the right and two ranks forward. A move of 5171 would mean that the piece in the indicated square moves two files to the right - if that piece is a king, this would therefore indicate a kingside castle. A move of 77882 would mean that the piece in the indicated square (a pawn in this case) moves one file to the right and one rank forwards, and the final character indicates the pawn's promotion to rook.