I'm thinking the Butcher cannon is not so much a new weapon, but a new set of rules for heavier autocannons, much like how the guard got the Exterminator Autocannon. Butcher cannons seem like vehicle scale autocannons, for Dreadnoughts and Defilers, maybe predators, while Reapers are infantry scale. It's less about appearance and more about the scale of the weapon.
As for no mark specific models, I see this thing as more a light defiler, and not a heavier dreadnought. It is Soul Forge/ Dark Mechanicum aligned first, rather than god aligned.