On this serial number:
5= 1955 model
4= Special Series
69= 4 door sedan
800678= production serial number unique to that particular car
The last part resembles a US body serial number. It may be the 678th 55 Special 4-door sedan built in the Oshawa, Ont. plant.
In cases where US-built cars were imported into Canada, these used the American-style numbering formats as shown on this website.