This item is a replacement seal for the Cooper Pegler 2000 Series lid that fits CP3 (20LT) and CP15 (15LT)
The rubber seal is essential to stop leaks and maintain tank integrity. Chemicals may degrade rubber - always beneficial to have a spare.
2000 series knapsacks are piston pumps - distinguishable by the central black, concertina-shapped column rising up from the centre of the tank. If you have a diaphragm pump such as the Evolution or Classic knapsack - the lid is different.