This is a speedometer held lightly by the tailstock live centre to show the bezel located in it's locking groove only. The larger aluminium block is then fitted to the back of the speedo and the live centre engaged with it for the rolling operation.
Speedometer sitting in the various holes to allow the block to fit snugly against the speedo back.
The speedometer just apart from the pushing block, showing the holes to engage the back of the speedo.
A light push by the tailstock to crush the speedo case against the flat rubber seal on the glass and ensure a water-tight seal.