Well that is strange. There is a difference between the two partnumbers but the older one should only be incompatible with the C9300LM versions.
This is the screenshot for the Cisco mode C9300 stacking cables.
![GIdenJoe_0-1729613419221.png GIdenJoe_0-1729613419221.png](https://community.meraki.com/t5/image/serverpage/image-id/40090i003223BEE7B6B50C/image-size/medium?v=v2&px=400)
This is also present in a Cisco community post.
![GIdenJoe_1-1729613541412.png GIdenJoe_1-1729613541412.png](https://community.meraki.com/t5/image/serverpage/image-id/40091iC82BB766FBDDFF3B/image-size/medium?v=v2&px=400)
So it might be that the -M variants also use a slightly different version of stackwise-320. I would hope a Cisco engineer would explain the difference between the two.