I have had the same question myself and I'm pretty sure it is This.

Here is a thread about repair and a link to pictures in Alan57's album that I found helpful.

Here


Eric
http://s1199.photobucket.com/user/sparky234/library/My%2057%20Chevy%20Stepside?sort=3&page=1
'57 Chevy 3100 (Little Red)
'81 RD350 LC - Fun, Fun, Fun
'83 GS1100E - Pocket Rocket