Hi,
IMHO I would measure the resistance in the coil/bell part of the circuit. Between TB-2 and TB-3. Then add a lesser resistor into the Lamp part of the circuit between TB-3 and the lamp socket. Approximately 10% below the resistance of the coil/bell circuit. This should help to balance the resistances between the two parallel branches. This should equalize the current flow and hopefully stop the sparks. If you want to even out the sensitivity or make the contact time between the handle and the wire to be longer then add a capacitor between TB-2 and the top of the coil. This should not only delay the activation of the coil but prolong its activation or hold time.

Hmmmm, Put a 100 micro farad capacitor in series with a 1okOhm resistor. The coil is involved also so you must measure the coil, add to that a resistor which equals 10kOhms between the two of them. Then add a 500 micro farad capacitor in series. I think s this should give you about a 1 second delay. I am digging into my brain from 30 years ago though so this could also erupt Mt. Vesuvius.