Porsche classic part this is an original ball-type bearing for the inner side of the early “small” 25 mm spindle   it measures: id 25 mm and od  62 mm  it’s marked “17305 on the face of the inner ring   tip  -- don’t forget to have a set of front wheel bearings in your emergency travel parts kit  

"Ball-type Inner Wheel Bearing For 356 Pre-a And 356a W/ Early Spindle"