JCB Wiring Boom Harness Part No 721/C1956 fits various Loadall Telehandlers Machines The JCB part number 721/C1956 is listed as a Boom Harness (sometimes described as P561 Boom Harness). It is an electrical wiring loom that runs through the boom section and connects components such as work lights, sensors, and auxiliary functions on certain JCB machines.