If it says "see pen case" on the enlistment record, it should have a number that follows, probably a 4- or 5-digit number.Often that is not a pension number, but a bounty land number.You can search for it at the Bureau of Land Management website (free) at:www.glorecords.blm.gov
When the main screen comes up, click on "land patents".Then click on the state to change it to "any state".Don't put in his state or name or anything else except in the section marked "miscellaneous" where you want to type the 4- or 5-digit number where it says "document #"
It will bring up a list of names of people who have land patents that bear that number.There will be more than one because land was issued under various Acts of Congress and the different series and different land offices are numbered independently.One of the names on the list should be your William.
You can click on the correct one to see an image of the patent certificate, which will name what regiment he served in (if it's for a soldier), so you can make sure you have found the right one.
The BLM website lists the original patentees of land purchased directly from the federal government, thus these land transactions are not recorded at the county courthouses until the land is sold to someone else.Each transaction at this site would have a small file of documentation at NARA.With soldiers, there is often a bounty land application file to go along with it (also at NARA).