Those are actually strips of tape, placed over the seams around the fuel tank panels, which weren’t airtight. The tape was generally coated with sealant, to make the seams airtight and prevented fumes and fuel from leaking out and smearing the canopy, or fumes from accumulating in the cockpit and creating a fire hazard. Perhaps someone with more knowledge on this specific point can complete it (or correct it if I’m mistaken).