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This object is called a "knife head." It's a sickle mower part. Parts like it in function are still parts of modern farm machinery including even the most expensive combine harvesters.
It has a ball on one end, which forms the ball part of a ball and socket joint which is necessary to move the knife back and forth.
Just as an example, here's part of a sickle knife with knife head, different, newer design. This one fits a Case-IH combine.
This tweet was supposed to go on this thread. Some days Jeffie don't do so good. https://twitter.com/homemadeguitars/status/1267190224003780610?s=19
Here's a video of the mechanism. The little wheel at the front, driven by the mechanism, is what drives the knife reciprocating action.
