Please see the attached photographs. The V is magnetic, and has a removable iron "U" shaped keeper. I assume it would be magnetically held to a shaft or a spindle.
Enst Winter & Sohn - Germany was a company which manufacturers diamond cutting tools. The text also shows that together with this tool also Winter diamonds need to be used (verwendet means uses). I couldn't find what the English translation for "Spretzfassung" is so it's not clear for me what the tool exactly does.
Beside cutting tools the company also manufactures tools for sharpening grinding dishes so could be also for this application.