Blank Nameplate, Circular, Fits between Coil and Nut

That's exactly right. It's designed to fit snugly in that space.

Purpose-wise, it is a blank circular coil nameplate meant to fit in that spot between the coil and the retaining nut on the assembly.

That's where it goes. The circular shape is made to sit between the coil and nut on Eaton Vickers hydraulic coil assemblies. Part number 565560 is the blank version, so there's no pre-printed labeling on it. That's useful when you're replacing a worn or missing nameplate and plan to mark it yourself, or when you just need the physical fitment covered.