Land Rover in the 1960s was famous for having stock from different suppliers - forgetting about it - finding it and then bolting it on what ever was on its way out of the factory. I imagine there was a similar working practice in the 1980s
Not an original type on series but looks like other aftermarket series tanks. The serial on the tank leads to a company who supply fuel tanks for boats!
Those clamshell type tanks were an extremely common after market tank. Widely fitted up to about twenty years ago. No idea why they stopped selling them.