Is the output different? Specs (sourced online) seem to be same on Mk 2 and Mk 3 - 0.4mV @ 12 ohms. This is a typical ratio for Shelter; their top "ultimate line" cartridges are 0.5mV @ 15 ohms which actually indicates a bit more coil is used (the reverse of most cartridge lines as you go up). Shelter also goes "against the grain" by reverting to aluminum cantilevers in their top line. Plus there’s the fact they stick with a plain elliptical on all but their top Harmony cartridge. No micro-ridges in sight.
Very interesting company that definitely doesn’t get much discussion these days. I fear they might be winding down, as the 7000, 9000 and Accord models suddenly disappeared from their website a few years ago, with no replacements. The Accord was only just introduced in 2014. And I actually prefer it to their top Harmony model - sad it was discontinued. It never got any press.