J,

The keyword being "usually." I remember the contortions that even you
(Shal) and Mark (offlist, through support) went through in trying to
make the threading happen correctly in several cases in my group.

I don't recall at the moment.

At some point prior to the message I cited Mark had added the "one-week" rule for messages without either form of citation field (In-Reply-To or References). I think that was to work-around various email clients, especially mobile clients, which fail to use or provide those either of those header fields.

Shal
https://groups.io/g/Group_Help
https://groups.io/g/GroupManagersForum