I find it hard to believe that they copied the word for workshop (presumably 工房) convincingly enough that it wasn't obviously written by someone without any understanding of how to actually write the language. It's extremely obvious when someone tries to copy Chinese characters without any understanding of stroke order, stroke pressure, etc. The way that someone would show how a character looks to someone without any knowledge (ie textbook form) and how they would naturally write such a sign is also different. You would be able to tell instantly that a non-native prisoner wrote it.

Actually, signs were also written right-to-left horizontally during that period but it's likely that someone showing them how to write on a piece of paper would have written vertically, so they would probably not even have the knowledge to know the correct order of the text.