You have quite a collection of cars. Thanks for confirming what I believe is correct. I have two 70 cars...one a 70 Skylark conv granny car.....2 owner and it is all original and it does not have any holes. The other car is a 2 owner 70 GS455 coupe and it is also all original...no holes in those fenders.
I believe the 71-2 cars got the holes...maybe Buick discovered a problem with welding the inner brace/hinge support to the outer fender skin and added dowel/pin/alignment holes to prevent future problems. Could also be a plant variation.
I have a friend that bought a 71 Stg1 coupe new and still owns it....it is all original and never been touched. I will check his fenders when I see him.
Thanks for your information.