Registering Older O'Reilly Books

I have many older O'Reilly books. My copy of Practical C is from 1991. I tried entering it's ISBN into the 'Register Your Books' form to see if any deal for getting a discount on a newer edition is available, but it came back as unfound. How far back can books be to be registerable? I have been buying new O'Reilly books for decades and would like some of them in ebook format, and to buy newer editions of some as well.
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