To me that is NOT a stringer but part of the floor board that knocks on and off with a block of wood and a hammer. Might require a replacement floor board to get that part. Keep an eye on Ebay for sure.
Stringers look like this, and are long enough to fit over all three rear floor panels, between the black plastic parts on the outside edge of the 2nd floor board and the stern floor board. In your second picture you can see the black plastic on the left outside. The third photo shows the wear marks on the aluminum floor boards where the strings rides.
https://defender.com/en_us/zodiac-in...stringer-z6452
Is one of them missing or damaged?