In the good old days, the 2 terminals for the primary winding were marked 'BAT' and 'CB' (contact breaker). Now, as I understand it, the 'CB' terminal was common to both the primary and secondary windings, so it completed the HT circuit to earth. If the connections were reversed, the primary winding would then also be part of the circuit for the HT voltage - thus passing HT voltage into the main wiring harness. Now the HT amperage is minimal, so probably wouldn't affect any normal electrical equipment - but I'm not so sure that applies when 'electronic equipment' (modern voltage regulators, etc.) are fitted. The modern system of labelling the primary terminals '+' & '-' gives no clue as to the internal wiring of the coil, and thus, which way round it should be wired.
Regards,
Eddie.
