Thank you for selecting Aerospace Aviation360 as your go-to distributor of part number 20307-21 found under Guide Valve Stem by Electric Boat Corporation. The National Stock Number assigned to this part is 4820016338476, and the Federal Supply Class is 4820 Valves Nonpowered. Our website is owned and operated by ASAP Semiconductor, a leading distributor of aviation and aerospace part numbers like 20307-21.