Oh dear, that is not easy to read or to photograph, is it?
Normally, rubbing a little talc over the mark can bring it up, but not against a white background.
Do you have any coloured chalk that could be used?
Or perhaps rub a soft pencil lead over it, sideways, like a brass-rubbing, to help?
Also, an image of the whole piece would help. Some of us might recognise the work.
Your image is grand, thak-you!