This is a Montbell waist pouch. It has variable capacity and folds up compactly. https://webshop.montbell.jp/goods/disp.php?product_id=1123984 The following is a description from the manufacturer: This unique pouch is designed to resemble a banana when packed. The capacity can be adjusted in two stages to accommodate the amount of luggage. Specifications: 【Material】70-denier nylon ripstop [urethane coating] 【Weight】109g 【Colors】Black (BK), Dark Green (KHGN), Yellow (MST), Navy (NV) 【Size】Height 10 x Width 28 x Depth 10cm (Length 20cm when fully extended) 【Capacity】1.9～2.7L 【Storage Size】Width 9 x Length 27cm 【Compatible Waist Size】Up to 105cm 【Retail Price】Approximately 3,500 yen