Dunno for sure, but the big plug might to allow the fitment of a heating element to prevent the coolant from freezing in seriously cold places like Alaska or northern Canada.
Oddly enough, I do actually posses such a British Playland heating element and it looks like it's about the right size to screw in there. (buggered if I know how or what such a part is doing in a warm climate like OZ, I got it in a job lot of parts) I believe you plug it it to your mains power, whilst the car is parked up over night. The one I have is 110 volt.......even more useless here in Oz as we use 240 volt mains here
Maybe some of our more arctic residing members might be able to shed some light on the subject.
"Keep calm, relax, take a deep breath, focus on the problem & PULL THE BLOODY TRIGGER"
'80 Triumph TR7.
'97 Ford Falcon Longreach 'S' ute,
'98 MG-F.
'83 Jaguar XJ6 Sovereign S3.