After it is bottled Brandy will not age anymore like whiskey so a new bottle is no different than an old bottle it is not like wine. Your bottle of Brandy made from pears of le bon Pere William was made before 1979 when it was not sold in the United States due to alcohol contents restrictions at the time.
Using my resources to look into your inquiry, I was able to find comparable sales that give your mid 1900’s Bottle of Swiss Brandy le bon Pere William a current secondary market value of $25 - $50 USD at auction or in a private sale.
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Alan C. Ransdell, AOA AM, CA