The mystical adapter plate of the Belgian jigsaw camo rucksack. Would you like to turn your rucksack into a Transformer? This will make the magic happen. You can attach these in place of the side pouches or underneath the pouches in case you want to stiffen them. However, the main purpose is probably to attach the side pouch to the Belgian vest.
According to the text on the product, the official name is Adaptatieplaat/Plaque d’adaptation. These were made in the late 1990s. 100% PA. Measurements 29.5 x 20.5 cm (11.6" x 8.1") . Weight c. 300 g (10.6 oz). The attachment straps are MOLLE/PALS compatible. Furthermore, they fit on a c. 5 cm (2") belt in case somebody wants to use these on a belt for some reason. NSN 8465-13-116-7295. These are also compatible with the newer Berghaus side pouches.
Used Belgian military surplus. These are in pretty awesome condition though. They might have slight surface dirt and such, but that won’t have any effect on usability.