(a) Watertight doors, except doors between cargo spaces, are classed as follows:
(1) Class 1 - Hinged door.
(2) Class 2 - Sliding door, operated by hand gear only.
(3) Class 3 - Sliding door, operated by power and by hand gear.
(b) The following types of watertight doors are not permitted:
(1) A plate door secured only by bolts; and
(2) A door required to be closed by dropping or by the action of dropping weights.
(c) Whenever a door of a particular class is prescribed by these regulations, a door of a class bearing a higher number may be used.