It sounds like a camlock attachment.?? You will need to count the number of the camlocks you have and the diameter of the camlocks and the diameter of the outside of the spindle that the chuck gets centered on.? These will help you find out which standard camlock you have on your lathe.
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After You need to buy a backing plate for your lathe then machine it to fit your chuck and your lathe.?? Backing plate diameter needs to match (roughly) the diameter of the chuck you want.
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Here is a good video on how to make the backplate ?
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If you buy a premade backing plate you can start from 23.35 minutes into the video.
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