The Cessna 188 is an agricultural aircraft with a 200 gallon hopper, usually for carrying pesticides. I expect that this had been used as an extra fuel tank. The standard fuel tanks take 37 gallons. My dad was an agricultural pilot in the UK and in the early 1960s they would fly Piper PA25's from the UK to Sudan to spray cotton. That was made possible by using the hopper as a fuel tank.