I think it isn't a bad idea to have a parts movement, as long as it isn't clapped out, especially for an heirloom watch that you may intend to pass on to the next generation.
Parts from the mid-500 series are getting harder to find, and one day donor watches will be the only resource, so having a parts movement is a form of insurance for the future.
Your Grandfather's watch looks pretty good, and with regular servicing, a little care and a ready supply of parts, who knows how long into this century it will continue to beat it's little heart away
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