I can upload some pics, but honestly, there's nothing to see!
If the culprit were readily discernible, I'd have it. Certainly I've spent enough time staring permanent disfigurement in the eye by putting my face next to the header to look in there as it runs.
All there is to be seen are some oily bits.
Absolutely nothing stands out.
What brand sigmoidoscope did you buy off fleabay?