Hard to find- NOS.  Child seat adapter needed to install an Easyfit style Yepp Maxi seat on a conventional bike rack.    The XL version extends the top surface of the rack to allow the use of panniers behind the child seat- allowing you to carry a child and your things, which is often not possible with a normal child seat.   Can be cut down with a hacksaw to use with older cargo bikes only compatible with the easyfit system.