Prior to the late 60s, Weber used thumbscrews to hold the legs in the bowl sockets.
The PAT PENDING and PAT number 3538906 that eventually got stamped on kettles refers to the Weber innovation of using the grooved friction system to hold th legs in place.
These details help folks to date kettles. Golly's cado is pre 68 ish!
This picture of the recent Imperial shows a pretty good shot of the thumb screws: