The '48 has one 12.5 gallon tank in the left wing and two 12.5 gallon tanks in the right wing. The story I've heard is that Cessna was reusing tanks from the 140.
It was not uncommon to see an additional 12.5 gallon tank added to the left wing. It just mirrors the right wing installation.
You do want to check your paperwork to make sure the mod was properly approved. There was never an STC, so it would have been done on a field approval. Getting a fuel system modification approved these days could be tough.