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Why did some Sud Aviation Caravelles have a very long dorsal fin stretching half the fuselage?

Initially I thought it's to keep the fin low to fit in existing hangars as a selling point, but the variant shown above (10B3 Super B) can also be found without this fin:

(Source)

There are two competing theories on airliners.net forums. One of them says it's fuselage strengthening for over-water flights in case of ditching. The other is a housing for an optional HF antenna. As far as I know, an old HF antenna was a stretched wire.

I checked the English, French, and Swedish Wikipedia articles, only the Swedish mentions it:

The aircraft was also equipped with a long dorsal fin (Google translate).

A Flight issue from 1956 does not show the fin in their cutaway (the article also does not mention it).