Because it was said to damage the seats of cars and the saddle of horses the rivets were no longer exposed starting in 1937. The covered rivets eventually became known as 'hidden rivets'. It was used till about the 1960s. 

Here you can see the rivet in the back. In the vintage denim world the price will change significantly depending on this placement. 

The L Company (Levis) patented the first rivets in work clothes. You could say that this was the beginning of the jeans history as we know it. Perhaps I wouldn't be making jeans today if the rivet was never placed on the first Levis jeans. This detail was seen until about 1937. 

Though it may appear the same, this hidden rivet detail could change depending on whether it was copper or an iron rivet.