There was a Programme on this
There was a programme on this and it's on youtube
features specially built 3d X-ray machine and 50 angle light to obtain layer and text detail not available in plain sight.
A completely fascinating programme that has had 3 or 4 runs on BBC4. IMHO it is worth committing the time to watch this from end to end
This goes a long way to understanding how this was constructed and how it works