DC-6 vent

[hawkone]

I have been looking at some great photos of the good old DC-6 and I noticed that most aircraft have a ram air vent for the cabin heater under the fuselage and it is quite visible. However there are some DC-6s that don't seem to have this vent. I looked at series A, B and C and they all seem to have this vent but one aircraft in particular stands out G-ASPA. There are a few others but this one comes to mind. I don't think it has anything to do with the fires the first releases were subject to. For that solution they moved the fuel vents. This really baffles me so if you know the answer I would appreciate it.
Thanks

[411A]

The vent is generally removed (as is the air cycle machine and associated heat exchanger) if the airplane is operated unpressurized.
Some operators did not do this, however, so...it depends.

[hawkone]

Thank you very much. I really appreciate you taking the time to reply.