It think this is an error in the parts catalog, but I believe the spring washers are incorrect for the rear mounting bolts of the seat base (item #9). Rear shoulder bolt #21 and washer #20 seem correct, but if the spring washer is installed, the id is less than the bolt shoulder od, preventing proper securement. #9 spring washer is also used with the bolt for the flip up base which has a much smaller shoulder od. That works fine. Am I correct in assuming the rear bolts should compress the washer and spring washer against the top mounting rail of the seat base? If so, probably should run by the hardware and just pick something out which fits.
http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet_Alt
http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet_Alt
