Classic/Antique Car Repair: Starter relay or related problem on 1956 Buick Roadmaster

Question
QUESTION: When starting my 1956 Buick Roadmaster, often the starter will stay engaged once the car has started and I have to open the hood and tap the starter relay to disengage the starter.  I have had the mechanic change starter relays and just last week they had to rebuild the starter due to damage from being engaged so often.  They thought they had the problem solved, but it persists.  Not everytime you start the car, but too often.  Any suggestions or advice will be great appreciated.

ANSWER: I have a question, is this car one that engages the starter swhen you turn the key all of the way over for when you press the gas pedal?

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QUESTION: When you press on the gas pedal

Answer
This system uses a switch mounted on the throttle body of the carburetor . when the throttle is depressed and there is no intake manifold vacuum (engine not running) the switch allows the starter to engage. Once the engine starts and intake vaxuumnis seen the switch is supposed to disengage the starter. I would start by checking the operation of that switch.