Mk.7 = 174 grain flat base projectile, cordite loaded
Mk.7z = The same as Mk.7 with nitro powder instead of cordite
Mk.8z = The same as Mk.7z except with a 174 grain boat tail projectile.

Mk.8z was designed for use in the Vickers MMG.

I'd like to read the story too. I'm a skeptic as the differences are subtle and the reasons cut and dried.