Mine is not that car but mine is also an S3 (coupe) with S4 wheel arches.
The front arches happened because only 3 months after I bought it secondhand in 1969 from Ian Walker I pranged the front and had to have a new nose section. Ian Walker's did it for me and changed the wheel arches to S4. It is possible that they could not get hold of an S3 nose part at the time and anyway it was in the days when no one worried about originality and concentrated more on making a good sports car. Lotuses Elans were S4s by then and There was obviously good reason for bigger arches so it was obviously good for my car too, right.
Later in 1982 on a major refurb by Vegantune of Spalding I asked them to widen the read arches too, and they fitted S4 ones.
See my main thread to get an enlarged picture.
lotus-elan-f19/getting-ogu-roadworthy-again-t26101-105.html#p189868