Thank you for selecting Aerospace Aviation360 as your go-to distributor of part number 504521 found under Filter Element Air Conditioning by Cardion Inc. The National Stock Number assigned to this part is 4130000618703, and the Federal Supply Class is 4130 Refrigeration And Air Conditioning Components. Our website is owned and operated by ASAP Semiconductor, a leading distributor of aviation and aerospace part numbers like 504521.