That was the key that the syntax was different in the expression builder and the render engine.
I like the trick with the shift too! I’ve been toggling over to the JSON, then making the change, and then toggling off again to get the calculated expression in there.
Good news is now that I’ve removed the quotes around the header, I can properly get the value of the various headers. When I incorporate this in the main rule, I’ll post it here again for @KeithTessler and others. Do you have a good way to share rules like this, other than just a copy/paste of the screenshots? That certainly serves the purpose.
Examples of the parsing working for various headers (from a Microsoft Automated reply):
