I would be surprised if all-737 Comair would acquire a smaller aircraft only to serve a single destination on a weekly basis. So we are most likely looking at a lease on a case by case basis.
Walvis Bay - St. Helena is 1.400m as the crow flies, so we are talking about an Embraer 135/145 or a CRJ200. Certaibly not the type of aircraft the locals need - not comfortwise, but to bring back goods as freight from visits to South Africa which probably is much more important on this route than almost everywhere else.