Washer, Tool Post
This washer sits atop the standard tool post on the 8x12 and 8x14 lathe.
The washer is 25 mm OD, 8 mm ID, and 4.3 mm thick.
In your instruction manual, this part appears on the parts list as:
|415||Harbor Freight 44859 Central Machinery Lathe - 8x12|
Various C8-US SIEG Lathe