The gear on the Centurion is the same layout as the 337; both legs swivel on a central fixture, rather than having actuators on each leg - so I'm guessing it would also apply to a 337. From my days on an old 337 at Cambridge, I seem to remember that the gear unlocks, then swings down under its' own weight to a certain point where it is locked down safe - or does my memory fail me? I think that would be the reason for the restriction, as you would want both legs to come down together to avoid stressing the central pivot.
But it was a long time since I worked on the old girl.....