It wasn’t practical to view the inside of a Paratoptic lock, with a mirror, because the large pin size and swinging plate on the front of the lock (I forget the terminology, detector cap?) blocked any view. The gap was only about 1mm.
Expert picklocks such as Pettes & Hall tried many different ways to get usable information via the keyway but with little success on the later locks from 1846 onwards.
Yale got around this by applying ink/black paint, diluted with linseed oil?, and applied with a artists paint brush which was cut at the end and stuck on a roughly made tin plate key. Using a series of tools with paper glued to them he was able to accurately determine the code of a lock. He then cut a wooden key to the measurements that he had determined. I have only ever seen bits cut for the 6 & 10 slider models only. What is more interesting is that he could still correctly decode the locks even if the combination was changed after the original inking. This would indicate that a wiper would not stop his system working.
I have only just recently found out that all his tools are not in one place.
