Lars, thank you for posting additional information.
(In reply to Lars from comment #3)
at one point I did find them, but no ".bak" extensions as indicated.
This finding is consistent with certain address books not being migrated to the new format, or not fully being converted.
All of this is too geeky for me! Out of 15 address books, I lost the contents of 7. The names are still listed, but contents are missing. These are the names of the missing: Birding, Birding Texas, Computing, FAMILY, Harte Trail, SAM, Vasa Lund.
So you have 7 address books whose names appear in the address books window, but those address books have no contacts listed in them?
Can you post a screen shot of the directory showing the file sizes all .mab and .sqlite files?
(FWIW I tried to find another report if this, but failed. https://mzl.la/3d2vxg2 )