The service manual for the Epson Stylus Pro 3800 is found here: https://www.manualslib.com/manual/820107/Epson-Stylus-Pro-3800.html This manual has an appendix with "Exploded diagrams" or assembly/parts drawings. These diagrams start from page 289. See diagram 5. Here is a crop from diagram 5. I think the mysterious plastic part is either circled in red or green: