MV makes sense, Meraki Video.
MR makes sense, Meraki Radio.
MS makes sense, Meraki Switch.
MX. What is the X meant to present?
I always thought MX because can function like a router and there's packets going everywhere. 🙂 Yes, I may sometimes imagine network architecture as diverging diamond intersections. Don't @ me.
The X in MX might refer to Extensible Threat Management, often referred to as XTM.
This essentially is a extension of Unified Threat Manage (UTM).