I have never seen a spacer used to calibrate the lens on a Rolleiflex camera. The washer shown does resemble the washer used between the shutter and the front standard, but the aperture assembly would not work properly and the shutter assembly would be loose if the washer were ommitted. The Xenotar lens does not have many elements, and installing them in the wrong order or in the wrong direction is highly unlikely. Ommitting a spacer between elements would either allow the unspaced element to rattle around, or break when the retaining ring was screwed on. Most likely the front and rear groups are mismatched, and the rear half is from a Planar.