Indeed, it is a good question. I checked several dictionaries for the word 'uncovery' but could not find it.
TheFreeDictionary has an entry for this but it does not clearly state anything on the page.
I think it is not widely used (COCA returns one result to the search 'uncovery'); MS word shows 'error' in the spelling and as I said, dictionaries including MM, OALD, Cambridge, WordWeb don't have such entry.
This seems to be a technical term or jargon in some context, not necessarily available on dictionaries. However, I had come across somewhere something like: "...leaving out an uncovery space on a glass.'
In addition, to make the verb 'to uncover' a noun, it would be 'uncovering' i.e. a gerund word. Uncovering is found in dictionaries.