The RTX1400si is a texture sprayer made by Graco for spraying orange peel, splatter, knockdown, and popcorn texture on walls and ceilings. Serial numbers 17H572 (RTX1400si) and 17P189 (RTX1400si RentalHD).
Here's a link that shows the size (and suppliers) of screw #17 Graco part # 120204.
If it's the wrong one, search through the parts list in the manual to find the Graco part number for the part you need and then search online for Graco (insert part number)