Take a look at the picture of these two thumb screw styles. I always assumed the style on the left was original, with the large single hump. If you go to the HW store, the kind you will generally find is the "three lines" style pictured on the right.
These specimens came off different "eras" of kettles, and I don't have a great way of knowing if there was a consistent style Weber used, if it changed, etc.
I was suprsied to find the older kettle had the three lines styleand assumed they were simply replaced at soem point. Now, not so sure. Thoughts?