These days a lot of tin-top bolts are, they ether have a rebated head or the hex is reduced - say a 10mm hex on an M8 where it would normally be 13. All in the name of weight saving. I was always impressed by the little weight saving touches on the Pug 107, which I guess is why it weighs 850kg in spite of having all the required safety gear that folk blame for the bloatyness of modern cars.