@knc1: CM is a more complex beast, it needs new 'code' [to handle new obfuscation and/or underlying code changes] for each patch-level version (i.e. for each new 'z' if we're using x.y.z as the version scheme).
And (usually, though that's not guaranteed to suffice), a simple special-casing for each new 'a' in x.y.z.a
.
Which is why 5.3.7.2 support was trivial, while 5.4.5 isn't
.
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As for the non-JP 4GB devices on < 5.4.5, you'd 'simply' have to get Kindlets working on those devices on FW 5.4.3.2, which probably involves something like the workaround that was done for the 3.X-on-DXg thing, since it appears to be a case of 'oops, forgot to put those device IDs in a DB somewhere, so we don't know them'.