Genuine Churchill tool made for Rover to line up the clutch end case that holds the plunger and clutch arm, not many people ever thought they needed a tool for this but its the true way of lining up the case to centralise the clutch bearing to stop wear- These are all SOILED and RUSTY as they have been in one of our containers for 20 years--Rust is free and can be cleaned in your time which is why they are cheap and does not cause any fitment problems. Useful paper weight.
The large end has a taper to fit on the crankshaft-the small end lines up your case with the plunger removed, for the release bearing to be central -put screws in finger tight first then you should drill and peg the case in its correct position so it has true alignment before tightening original screws to secure. The small end also will show any drastic wear in the clutch case where the plunger sits which can cause clutch problems.