Sextant Parts are used to create a complete Sextant. Given that the final item requires no skill or other items to create, the two step process seems rather strange.
Adding to the oddity, there are two places the parts can be found on the tinkering menu: Under "Parts" and under "Assemblies". The "assembled" version is more expensive in both time and resources.
From "Parts" Menu
- 4 Ingots
From "Assemblies" Menu