CPV code 42121100 designates Hydraulic or pneumatic cylinders in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
42121100 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 42121000 (Hydraulic or pneumatic power engines and motors).