Those small plastic pieces at the upper rear of the fender Ford called a "Front Fender Filler". They were production installed until 6/1/71 on the '71 Mustang and Cougar, after that date they were no longer used. Good used ones can be a difficult find since a lot of them have deteriorated over time, and were not installed for the entire '71 production run. Part number for the right side is D1ZZ-16570-A (ID# D1ZB-16570-AA,AB,AE) Left side D1ZZ-16571-A (ID# D1ZB-16571-AA,AB,AE). As you can see there were three suffix number changes on the engineering number but the part number never changed. For some reason, Ford continued to use a similar filler at the rear of the front fender for the 72-76 T-Birds.
As knn posted, there is a repo version available now, but since the part is hawked as for the Boss 351, the price reflects such. If you are working on a concourse level car, or just absolutely have to have them, then you can dig into your piggy bank for the 255.00 + shipping. Ford felt they were not necessary since they were discontinued before the '71 model year came to a close. My early build '71 Mach 1 (9-21-70) has them, but probably won't be replaced if they ever become damaged. But at least you do now have an option for a part that wasn't available until recently.