Thank you for selecting Aerospace Aviation360 as your go-to distributor of part number 4B3-3 found under Retainer Electron Tube by Birtcher Corp. The National Stock Number assigned to this part is 5960000717486, and the Federal Supply Class is 5960 Electron Tubes And Associated Hardware. Our website is owned and operated by ASAP Semiconductor, a leading distributor of aviation and aerospace part numbers like 4B3-3.