A rare two colour gold rectangular wristwatch with duo dial, Rolex, 'Railway Prince' 1931

9k, rectangular nickel lever movement,  Rolex Ultra Prima, 18 jewels, mono-metallic compensation balance, timed to 6 positions, silvered duo dial, arabic numerals, blued steel hands, subsidiary seconds, seconds hand lacking, rectangular case, stepped sides in white and yellow gold, hallmarked Glasgow 1931, wire lugs, case, dial and movement signed