Looks like the Everpure one. They are pricey, and not as easily found due to the fact that they are quite old. Google "Everpure" and I think it's a V400 or V500 the number of the cartridge.
They sell for around $35 plus shipping (I don't know any walk-in stores that stock them).
It might be time to look for a more modern one, with easier to find cartridges...
In reading a history of the ownership of Airstream, within the Beatrice years, they mentioned that Culligan was also once a Beatrice acquisition, and the Everpure was a division of Culligan, so it's not surprising that they were once supplying Airstream with filtering systems.
“Courage is being scared to death, but saddling up anyway.”