Tony T, the Robertson ERFS tanks (we called them “Bob Tanks”) were only fitted when we needed more fuel than the standard internals would carry (3000kg in the “thin tanks” and 6000Kg in “fat tanks” aircraft). Each Bob tank is just over 2000Kg of extra fuel. They were certainly battle damage tolerant, and IIRC, to a higher standard than the aircraft tanks as the fuel cell was in the cabin with the pax/freight. The normal tanks were certainly self sealing up to 12.7/.50, and they also self-diconnected and sealed themselves to minimise spillage/fire in the event of a heavy landing. There was also a dry-bay fire suppression system (“Cobra”) that would fill the gaps with inert gas to attempt to snuff out any fire before it started if it detected a round impacting a fuel tank. The major difference between a modern and vintage aircraft is in full system certification. Whilst modern technology and materials can be introduced, ultimately old aircraft designed and built to a price point with only short life expectancy will struggle to ever meet modern civil regulatory standards, hence why you will sign a comprehensive waiver to fly in one….