Unfortunately, the K25 retourne insert did not work for my 2003 sellier box Kelly. Not sure if it could be because of leather type or year variations in size, as others seem to have had better luck (I did have the Kelly authenticated by bababebi, who owns a Kelly in the same specs). The insert was too large for the Kelly, and it buckled whenever I closed the bag. It’s hard to see in photos, but I did notice that the corners pushed on the bag a bit, showing slight indentations on the outside of the bag. It seems like it would work well for a bag slightly larger on all sides, and I would recommend it for a K25 retourne for sure.
I actually made my own insert using couple pieces of felt, which fits better although it’s nowhere near as pretty as Maitai’s. I found that an insert measuring 8in by 3in at the base and tapering up to 7.25in by 2.25in fits very well, but seeing the variation in size for K25s, I would double check on your own Kelly if you would like to do this.