This is a 2nd Hand Plastic Seat Pad Volume reducer, sometimes referred to as a Spirit CTR Backrest. It’s purpose is to move the paddler’s sitting position forward closer to the rudder foot controls.
This screws into inset brass threads on the CTR deck.
It is often replaced by a backrest or backrest with seat pad which accomplishes the same thing with the addition of actual back support and under bottom comfort. Back support will aid especially for those with tighter ham strings. for greater support and flexibility. These backrests can often just be clipped onto the kayak in seconds, however if the appropriate fittings are not present in your model they are easy enough to add.
Screws not included