I went back to that bookshop today, asking if I could take a photo of the pendulum. Here it is and the woman in the shop today told me the clock wasn't made in 1765 like I was told on Saturday, but made between 1800 and 1825, so not quite as old as first thought.
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Apparently a lot of the weights on old long case clocks are not originals because being made of lead they were often melted down for scrap when times were hard.