Hi Andrew
The nut on mine is holding everything together, I have a feeling something probably wants to leap out from inside.
There is a loose pin on the mounting face and 2 pin holes marked L and R, I imagined this unit "interacted" with and tripped the saddle drive, but from your comments it's perhaps nothing more than an indexable set of stops that mount on the saddle, and a bed stop.
Rather than the other way round where the indexible bit is on the bed.
And no I do not have the bit that mounts on the bed, come to that I don't have the lathe either, It was just one of those useful looking bits that followed me home.
Phil