Theo Clarke wrote:
My understanding from museum and gallery folks is that the photography is a creative act. This is substantiated by the variations that are possible in the representation of the object (if only variations of intensity and hue). In the case of the page with the ruler and scale, the placing of those elements is a matter of selection so the image involves demonstrable creativity. So, the image depicted on the page of the book is out of copyright but any reproduction of that image can be copyright. Most museums have a fiduciary duty to their trustees to protect the assets of the museum; this includes intellectual property.
The polite expression for what they have been telling you is "bullshit".
The minor variations that you cite are often of no consequence to the user who can remove the so-called creative effects by changing browser settings or cropping out the ruler. This should effectively remove the "copyright" material from the picture.
Ec