Very belated apologies here. I didn't respond because I had a stretch of many months when I could only access the forum in read-only mode because the back-end software it uses had been updated and my browser was not supported. I'm glad @ccstone and @Zabobon were able to help.
All good. I've moved away (again) from filing in Mail back to filing in Finder. Thank you for completing the exchange.
Someday Apple might expose the Finder’s filing system within Mail and I'm not holding my breath for that or any other innovation 
.