Don't know if it helps any but catalogue says C, CC, F, M, M2, S2, S4, X, X2, 40, 401, C400, C600 used same packing nuts. No mention of M4 anywhere, but assume it also has same packing nut since transition from M2 to 40. Must be water pumps / parts around for that many models.
Our C400 / C600 would be same as the Beacon, Flyer and Ace Continentals if you can even find parts for those.