Andrew, while I cannot answer the question directly, HP would often describe the options by combining options together into one option no., ie, 003 and 050 = 053, however in this case I think it maybe a personality. ?There is somewhere in the menu structure a listing of the options installed in the unit. ?A single purchaser may have requested an option package designated as R53 (ie. radio testing personality). Are there any tags identifying the previous owner? The 8590 series was the first series of HP SA¡¯s that were easily customized by software/hardware options to get the best possible fit for your testing needs.
