The Talking Machine Forum — For All Antique Phonographs & Recordings

Anyone know what this is, I've not seen one before

Very cool. Thanks for posting. I've not seen this specific one, but here is another from the Utica Crane Company that serves the same purpose. They both work on the principle of increasing friction against the rotating mandrel stopping the rotation.

They seem to be made specifically for the model B Home (and later), as the model A home does not have the extra adjusting/support mechanism sitting on top of the half-nut bar.
And the stopper in real-life:
Shawn