For 09 and Newer Touring Models Mid-Frame Air Deflectors attach to the frame below the seat and direct heat away from the rider to improve comfort.
Difference in part # is that first part number 58170-09  was part of a OEM Tourpak Kit back when the bike was bought new but never fitted. The second part number is the number you would use now to order the part at your HD dealer. Hope that makes sense.
Original box has a few rips in it but the product is perfect.
Features
Designed and tested for severe duty and extreme temperature applications
Details: Fits 09-later Touring and Trike models (except 23 FLHXSE and FLTRXSE)
Not compatible with 17-later models equipped with Cool Flow Fan.