If you "zoom" (Ctrl-Alt-F1) on that page, you should be able to search for all available fields.
On the "Purch. Rcpt. Header" table, there's the SystemModified By field. But, it's just a GUID.
I'm guessing you'd need to link to the "User" table to get it to display the corresponding user's name.
Unfortunately, I'm not a developer so I can't be too much help beyond that.
