There should be a warning label on the 501s-- these are not for everybody.  The button fly is extremely awkward and difficult to work, especially if you are in a hurry or have the slightest problem manipulating too big objects through too tiny holes in stiff fabric.  This is an awful product for anyone who is overweight, has arthritis, poor circulation in the hands, possesses pudgy fingers or has wrist problems.  I would also recommend staying away from this product if you have bladder problems or are drinking heavily.