The 774SM was a more affordable variant. It is a very good arm IMO, not as good as the Zeta, though certainly Ittok-level, albeit with slightly different strengths and weaknesses. My LP12 trajectory was Ittok, 774SM, Zeta, so I know them all pretty well. The 774SM is around the same weight as an Ittok too, maybe a hair lighter, so very easy to get a nice even bounce out of the LP12. The weight of the Zeta makes this much harder as the rear spring ends up more compressed than is ideal. It is doable though and I’d not let that aspect deter. Some of the best sounding LP12s I’ve heard have had Zetas.
Never heard a Mechanic, though it was as I recall the final GB Tools design and I suspect brought the experience from the other two and took it further. As I remember the Zeta was first, by a long while, then the two Mission appeared around the same time at different price points.
As always with tonearms, especially from the dreaded ‘Linn tight’ era, I’d buy on condition over and above anything else. That and be wary of sticky bearings in the Zeta, they have a reputation for it which implies a lubricant drying out. My old one is still working fine though (a friend has it on my old LP12 with a DL-103, might even be one of my old 103s! Nice to hear it now and again via his Gale 401s or LS5/8s).