ISBN 13 numbers on a book published in 1981?

A friend wrote a book back in 1981, with Interweave Press publishing it. I did a search to find out if it's still available or 'sold out' as a book with that publisher and saw that it had two ISBN numbers. Wikipedia tells me the ISBN10 was the standard, and the ISBN 13 is the new format after 2007 for new books. Hers was not re-issued and she has not had a royalty check in 10 years from them.





I'm wondering if the new ISBN 13 number is a standard update for out of print books, or if the book was reissued and she has not been paid by them. She sort of dropped out due to medical conditions and thought the book was now out of print, but I'm not so sure. She's dirt poor and I don't want to see her ripped off.





I sent an email to Interweave press asking if the book was reissued, but haven't heard back yet. Anyone know if a publisher would continue to publish a book without paying the author?|||ISBN13 is backwards-compatible with ISBN10 - you can convert a 10-digit ISBN to a 13-digit one without having to allocate a new number. It doesn't necessarily mean that the publisher has reprinted your friend's book.