renaming the file won’t help.
the version number is gonna be read internaly, inside the firmware file.
i havn’t tried that though so it may work but its so unlikey i don’t see the point.
in my last comment i mentioned Rockbox as a hint 
That may be your best possibility
If rockbox has a restore original firmware feature
tha that might help for an older version ??
again i havn’t used Rockbox so i don’t know.
just trying to offer my 2 cents…
personaly i see no advantage to using older firmware versions
i have been upgrading my FW as they have been released
and never felt the desire to down grade… i only tried doing it once
for testing purposes (related to a clock time bug)
current version is probably best in my opinion
edit:
you said “I can’t get Rockbox run correctly.”
maybe focus on trying to get what you got to work rather than changing it ?
