I haven't seen any captive washers like that, only star washers that are a tight fit on the screw and just don't drop off. I think with a little more effort, since they're slotted, they could be removed and replaced with star washers. Pry the end at the slot up so it engages the nearest thread and then unscrew them. Clamp them in a vice between tip and head so you don't stab yourself with the screwdriver holding on to them by hand.
