I could be wrong but that washer doesn't look like brass or bronze to me. Also looks too small for a drain plug washer, those have a larger inside diameter and aren't as wide (face), they're usually copper or aluminum.
Are all the washers used for the bolts that fasten the lathe to the base accounted for? That looks more of that size & it's not uncommon for a different or stray washer to have been thrown in the box. I don't recall having one those color washers but I do recall having extra hardware, same for my mill.
Speaking of the base, if you haven't assembled it yet, it's best to apply some sealant like RTV silicone between the base, chip pan, & lathe before assembling as well as on the bolt threads
There are 2 types of foot brake pedal linkages, I have the 1 piece type and the 2 piece type seems to be more common. Not sure if the 2 piece type uses a washer like that somewhere.