I don't think there is a difference. Here's sylvania's description:
3157A / 4157A SilverStar Signal Light
High Performance Signal Light - Dual filament for both signal light and parking light - Irridescent when off, Amber when on - Wedge Base - part number: 3157/4157A ST
You can see the product number is 3157/4157AST
I picked up mine at Walmart...amber...love the look...
