A Friend in the USA who has a HF, Ed Levin, asked could I post an answer regarding this vehicle for him, under the circumstances it's obvious Ed's comments are relative, so here they are.
"There are many transitional-spec Fulvias, but this certainly doesn't sound like one of them. Your instinct that this is an S1 with a later front end is exactly right, and accident repair would indeed be one plausible explanation though there could be others.
It’s also very unlikely that this is a genuine, unmolested HF. You'll want to confirm the chassis number, but a real 1.3HF would be chassis type 818.340/341, and it would have engine type 818.342—not type 818.303; the latter is correct for an S1 1.3 Rallye S and all S2/S3 1.3S Coupes as well. Plastic wheel arch extensions were only used on the 1.6HF–not the 1.3HF, so these have also been added after the fact. But, in short, it’s clearly not correct as described.
The one thing that isn't very worrying in itself is the date of 1970. Fulvia Chassis Plates don't carry a build date, so it's not unusual for the cars to be registered with a date somewhat later than the actual build date, which in the case of the 1.3HF would probably be 1967-1968".
Brian
8227

Please note, amended at Ed's request.