Its real easy. Just open it up and it should make pretty good sense.
Here's the one from my '51 - http://rustyfiftyone.com/2012/04/11/snot/

The hole can be plugged with a grease plug/cap available at any hardware store. I got a stainless cap from Ace that was less than a dollar.
They look like this - http://www.classicparts.com/images/91-205.jpg - and are identical to the ones from the regular online parts vendors.