Model 53E is a special designation for Models 520 and 52H converted to 53H functionality (33 MHz Processor and 33 MHz memory) as on-site upgrade. See US Announcement Letter 192-005Archived 7 October 2012 at the Wayback Machine
Model 58F is a special designation for Models 530H, 550 and 560 converted to Model 580 functionality (62.5 MHz Processor) as on-site upgrade. See US Announcement Letter C92-016Archived 7 October 2012 at the Wayback Machine
Model 53E is a special designation for Models 520 and 52H converted to 53H functionality (33 MHz Processor and 33 MHz memory) as on-site upgrade. See US Announcement Letter 192-005Archived 7 October 2012 at the Wayback Machine
Model 58F is a special designation for Models 530H, 550 and 560 converted to Model 580 functionality (62.5 MHz Processor) as on-site upgrade. See US Announcement Letter C92-016Archived 7 October 2012 at the Wayback Machine