+1 on on using the 'z' digit JUST for emergency version fixes for ALL releases. If it is bad enough for a emergency patchup... those that
have downloaded need to get some sort of notice (besides MR)
Since Calibre release Version 1, this digit has been 0, so why not?

If the upgrade is for currently version (matches x and y), Make the notice Red or blink to inform them of a emergency patch. The notice stays normal for older installed versions