What is this called and what does it do?

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[font "Times New Roman"][#000000]What year is that motor? We may be able to figure this out through the parts lists or diagrams. [/#000000][/font]
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[font "Times New Roman"][#000000]Also, look at the diagram on this link:[/#000000][/font]
[font "Times New Roman"][#000000][url "http://www.sterndrive.info/outboardmotor/id10.html"]http://www.sterndrive.info/outboardmotor/id10.html[/url]

By your picture, it does not look like the prop spacer. Try and see if it matches the cap like thing in the back of the prop.
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