It should as the HBA, X6541A (375-0006), is listed as a supported controller for both systems: http://sunsolve.Sun.COM/handbook_pub/Systems/E420R/components.html#SCSI
http://sunsolve.Sun.COM/handbook_pub/Systems/A1000/components.html#HOST
The A1000 manual was probably published (April 1998) before the 420R was released (Nov 1999).