I've just bough two incomplete 13-050 plough planes to combine into one. I noticed machining(is that the right word?) is a bit different between the two but also one has a screw next to a handle. whats it for?
Position the sliding section where you need it, using the thumbscrew onto the front rod. Then tighten that screw against the main body, to lock it up solid.
The Record 050 has the same feature.