Sorry to be the spoil sport.
One idea would be to check: if the message is addressed to only group then go with [Reps Leadership] Test for Discourse. Otherwise use [Group PM] Test for Discourse. I don’t like the variance, but it’s the only way I could think of salvaging @LeoMcA’s idea.
Another idea is to ditch the usage of “P” entirely and not try and disambiguate by the number of recipients a message has. Just call everything messages:
[Message] Test for Discourse
In my summary of this issue this modifies (1). Which means that (2), clarifying recipients in the email, needs to be very clear.
Basecamp gets by with a byline in the footer. I’d start there. Let’s roll it out to our groups and see what catches fire 