It’s a Vintage 45rpm Record Adaptor!
Most of us know 45 rpm adapters as small plastic or metal insert that goes in the middle of a 45-rpm record so it can be played on the LP or 78 rpm size spindle of a turntable. But there are also larger adapters like the one that fits over the entire spindle, permitting a stack of 45s to be played. Who’d have thunk?