Retested with all three SPL meters. One old RS , one new RS and Galaxy 140.
I didn't bother with the ECM8000 today because the Galaxy is so close.
C-ticked and not ticked for each meter in turn.
Correct cal.files loaded for each meter after clearing cal.file each time.
I downloaded the latest cal files from the download area.
The two RS meters match well despite the 10-12 year age gap. (Upper green curves)
The Galaxy 140 and ECM8000 match within a hairsbreadth.
Neither pair agrees with the other.
I started out hoping this test would be a useful comparison to save people wasting money upgrading their test equipment unnecessarily.