We conclude that the pinhole is unlikely to affect any science data. Additional pinholes are visible also in the same filter in WF2 and WF4 at the level of one percent or less, but they are much less prominent in the pipeline flat fields and are also unlikely to affect science observations. A pinhole affecting counts at more than 10 per cent level is visible in the top part of the frame. The pinhole center is outside the field of view.
Differences are present at all spatial scales from the whole chip down to diffraction features.
In order to verify whether these differences were more geometrical in nature or dominated by a pixel to pixel variations, we have smoothed the flat field ratios with a 3 by 3 pixel gaussian box. The variations remain essentially identical. Once the smoothed frame is subtracted from the original the residual frames appear dominated purely by noise.