Tom,
Unfortunately there is no easy way to find the underlying vault filename for a known Windchill item - it is not visible in the Windchill UI.
This is why you can't find the hexadecimal filename that you are looking for in the vault filesystem on your system - you are not looking for the correct filename.
You need to run a SQL query that joins several different tables to convert between a particular iteration of a Windchill document part name to the underlying vault filename - sorry it is just not that simple.
That is why I suggested asking PTC tech Support or your VAR to help you or provide you with such a script because it is a reasonable complex query to dump all this information from the database.
I just gave you the information in the hope that you would at least understand the process.
Rgds
Gary