Letter sent on 12 October 1804 from Barcelona to Port Maurice (Porto Maurizio). Endorsed via Girona. Barcelona was suffering from yellow fever. It was douse in vinegar for disinfection. I suppose that the disinfection took place near Perpignan. Other option it is that was disinfected at Barcelona. Prof.Guy Dutau mention in his book that during 1803-1804 yellow fever epidemic, mail from Spain to France was disinfected in Perpignan area. On front faint departure cachet "B / CATALVNA".
I agree with you, this letter was most likely disinfected at/near Perpignan when crossing the border into France.