Epson PRO Reference Manual page 24

3. Rotate the paper thickness lever clockwise as far as it will go.
4. Press the Alt button. Then press Pause.
5. Close the printer cover.
Loading the paper tray
After adjusting the paper thickness lever, follow these steps to
load the paper tray:
1. Remove the output tray and remove any paper from the
paper tray.
2. If you are printing on envelopes, pull back the paper tray
extension. Then remove the rear edge guide, turn it around,
and reinstall it as shown.
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Paper Handling