Looking for the ignition box on your Honda CB125E? The easiest way of getting to it is removing the fuel tank.
Once it is removed, near where the bolt was you will see the CDI box hooked onto a frame protrusion:

It plugs into the wiring harness at the front and on the underside, if you're wondering when you put it back:

Connectors:

This one has the following marked on it:

The part number for the CDI box is 30410-KVC-681, and the part number for the rubber cushion that goes around it is part number 30401-KVX-601.
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