I have come across "Enterprise" labelled 5-Series models a few times. There were other Smith-Corona models that used the same name too, so I suspect it was a model name that was specific to a certain retailer of Smith-Corona typewriters. The same practice is still in place today: many large retailers will order models that slightly differ, even if only in name, so that it becomes a store 'exclusive' model (and can make price comparisons impossible).

​Is it rare? I really don't like that word as it's often over-used in typewriter collecting. I don't consider it rare, but it's certainly harder to find than the machines that used the regular 5-Series model names. Personally I wouldn't pay more for an Enterprise than any other 5-Series model given that there's nothing special about the machine other than its name.