Moved to support forum.
Answer of your question is dump your alternative flash partition to USB driver and inspect it under hex editor (of mine).
Wiki has some articles that explains manually dumping flash. You only needed to DUMP flash area, NOT write. You needed to use alternative flash region. You can find via in couple trying...
Also you don't needed to write exact version. Only compatible format of previous firmware is enough. Once you reverted back, you can also reflash older version to make string proper.

Thanks.