For DRM'd non-dual KF8 ebooks using .azw3 should work just fine. The question I have is what do we call the non-drm, non-dual KF8 ebooks both internal to and generated by Calibre?
One idea is internal to Calibre enumerate the .mobi extension in some fashion:
.mobi6 - means an older mobi file (< 8)
.mobi8 - means a non-dual KF8 mobi (viewable on only newer devices)
And then have a conversion preference that will determine whether people want mobi6, mobi8, or dual versions when converting to .mobi?
Either that of have Calibre only ever produce dual joint mobis when converting other ebooks to be mobi?