I remember seeing an Auster with a Lycoming engine and an underbelly extra fuel tank. .
It was likely Auster 5 G-ALXZ which was in the ownership of Michael Somerton-Rayner from Sept 65 to Jan 69. He brought the aircraft back from Germany where it had been on the German register as D-EGOF. The aircraft is still airworthy and flies wearing military marks NJ689......
Take a peek at some ""pics "" here:-
https://abpic.co.uk/pictures/search?..._fields=%5B%5D