What @P49Y-CY said is true. Back in the day, new owners often opted for the aftermarket mats with the snake. I was one of those. The original aftermarket mats had a smaller snake than is on the mats available today. The snake also was more offset to the left on the driver's mat, and lower. The mats you find today have the (larger) snake a bit more centered.
OEM Mats
Aftermarket "preferred" Snake Mats (no longer available with this smaller snake positioned more to the left) - Original Part No. 1R3Z-6313300-BAB
Aftermarket Snake Mats (now available at most Mustangs parts shops)