JanMM

I poured myself a cup of coffee and then went out to the garage where I took a look at an old non-shimano axle set. On this set, the closed side of the retainer (left on your photo) faces the spindle bearing surface and the open side faces the cup. When oriented correctly, the ball bearings will roll freely on both bearing surfaces. That's the key. Before installation, try it both ways; one way should be smooth-rolling while the other may involve retainer rub on a bearing surface.