I found a new option.
If there is no isbn available, but you can find a 9-digit number, it is possible this is a pre-isbn. A sbn-number.
Adding a 0 in front of this number creates a valid isbn-number.
Quote:
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The original standard book number (SBN) had no group identifier, but affixing a zero (0) as prefix to a 9-digit SBN creates a valid 10-digit ISBN. Group identifiers form a prefix code; compare with country calling codes.
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WIKI-isbn
E.g. in my library:
DT book used for version 1.1 reference: SBN 4260 5012 6
To get a valid isbn add 0: 04260 5012 6