So we have two pieces with mixed characteristics.
Practically identical boxes.
M piece have later rosette (as expected) than t piece.
M piece have more rich red "eyes" (as expected) than t piece (the later have more carrot-like enamel shade - as expected).
M piece have later type of hook on the ribbon (as expected) than t piece.
M piece have earlier style of reverse inscription (quite unexpectedly!!!) than t piece.
All others characteristics are practically identical...
And of course not punched, but engraved M
Hopefully this liitle case study clearly shows how difficult is to date sacred treasures only by the shape of cental mirror.
Here comes another 6th class with engraved M mark with exactly the same characteristics.
To consolidate our results ;)