All 8591C instrument shipped from HP/Agilent with the 85721A CATV measurement personality installed in them along with a memory card that had the software on it so that it could be reinstalled if necessary. Of course, over the years many of these have been lost.
Unfortunately HP/Agilent never provided a way for the user to download the software or load it into the instrument any other way that via a memory card, so I can't imagine you being able to restore it that way. We also don't have the ability to provide replacement cards any longer, so your only hope of getting one of these would be from a third party vendor or possibly someone here on the forum that has an extra. Of course, there are many different versions of this CATV personality, since there are many different TV formats around the world, and most use a different version of the 85721A (NTSC, PAL, SECAM, etc...) so be careful if you are going to purchase something...
Another possible solution may be to contact the Agilent service center near you to see if they might be able to load it for you if you were to send it into them, as they may still have a card for doing so when a servicing might require them to clear the memory of an instrument.
Unfortunately HP/Agilent never provided a way for the user to download the software or load it into the instrument any other way that via a memory card, so I can't imagine you being able to restore it that way. We also don't have the ability to provide replacement cards any longer, so your only hope of getting one of these would be from a third party vendor or possibly someone here on the forum that has an extra. Of course, there are many different versions of this CATV personality, since there are many different TV formats around the world, and most use a different version of the 85721A (NTSC, PAL, SECAM, etc...) so be careful if you are going to purchase something...
Another possible solution may be to contact the Agilent service center near you to see if they might be able to load it for you if you were to send it into them, as they may still have a card for doing so when a servicing might require them to clear the memory of an instrument.
Best regards,
Mike