Porsche classic part 2 required this is an original ball-type bearing for the outer end of the early “small” 25 mm spindle   it measures:  id 20 mm and od  52 mm  it’s marked 17304on 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 Outer Wheel Bearing For 356 Pre-a And 356a W/ Early Spindle"