MCB tripping at Fruit Machine startup

macman

Plug it into another socket on another circuit (use temporary extension cable if necessary). If it still trips, it's not the circuit, suspect the plug or socket.
Then try different socket on same circuit, if it still trips, suspect the plug or the machine itself.
Please get the CU re-labelled and the gap on the front filled ASAP as previously advised-it's potentially lethal in that state.
Personally, knowing now that you have a second hand CU installed, and given the indifference of the installer to labelling it properly, I would want the whole system checking over, as it smells strongly of 'cowboy installation'.