No part number to help with. But maybe it's in a manual? If not, I'm sure any Grady dealer worth their salt can get you what you need with just a phone call. Otherwise, chances are the ball is a 10mm - you just have to look at the gas spring to see what the poundage is. Then measure the closed and extended length - it might even say that on the spring (12-20, for example... 12" and 20"). With that info, you can get springs at any marine store. You can double check the ball size by taking one off - either the ball or the spring - and either measuring it or taking one to a store to size it up.