chrome ignition box: high voltage at the outside?

And the coil negative terminal will have high peak voltage too during spark. Typically the heatsink has a coil driver transistor that is insulated from the heatsink. The transistor collector or drain has a connection to the coil negative terminal. Careful inspection may show the typical TO-3 package on top of heatsink with a plate, and what looks like fiberglass cloth, or mica sandwiched between.

As a side note:
Modern coil driver semiconductors have a built-in clamp voltage called avalanche protection. While the protection works well in the short term, long term damage may occur from running with a pulled spark plug cable. Normal spark action clamps the voltage, with the open spark cable, the avalanche protection comes into play, and the repetition builds heat and damages the driver.