Last weekend we bought our first T25, an air cooled Devon Pop top. It's got a few little niggles we need to sort but we're happy with it

Today I was looking over it and noticed behind the rear wheel arch on the driver's side looked like a tank that had some rust but also looked like it had wet on it? At first I thought it was the fuel tank but looking online I think that's closer to the front? Off my pretty rubbish description does anyone know what it is?
We're new to all this but think we'll be on fast track to learning!
TIA
1981 2.0 Air-cooled Devon Pop Top