There are three types of coolant reservoir caps used on 33s.
The earliest versions have the metal cap mainly before 1992.
Later, depending on model version, the metal caps has been replaced
by plastic caps with and without rubber pipe (the reservoir tank is
changed also.) All the caps equipped with a pressure relief valve.
Caps with pipe are directing the excess pressure (boiling coolant) away from
the engine compartment. While the simple plastic cap is releasing the pressure
on it's thread, so the boiling coolant will be discharged on the top of reservoir
tank. This is unlucky because some models have the
ignition coil or ECU mounted near the coolant reservoir tank, and these parts
are sensitive to spilled water/coolant.
Reservoir tanks with metal cap also have a pipe, but it's connected directly
to the tank's neck, not to the metal cap. They directing the stuff away from
the engine compartment too.