Corny Keg Lid Styles - Why did mine leak?

I was lent a pair of Cornelius Kegs (pin lock) which I converted to ball lock before use and replaced all the O-rings. I was pressurizing a keg yesterday and despite my best efforts - could not stop leaking. Ultimately, I used the lid from the second keg to solve the problem.

What I found is that the leak was through a sliver - a scratch in the top of the lid. Which leads to my question. There appears to be a raised surface in the center of the lid. When it was formed, a dimple was pressed in from the bottom and (I assume) the raised surface was machined to a flat surface. Something would rotate in contact with this raised surface and ultimately scratched a hole. What was the purpose of this raised surface? None of the pictures of lids that I have found share this feature.

While the b r a n d of Keg is Cornelius, the b r a n d of the lid has a Firestone logo - as shown below.

I went to the trouble of taking photographs, but this is site refuses to use them in IMG tags or even use the URL as plain text.

 

I think I have found the answer, stamped on the lid itself.

http://www.google.com/patents/US3186577

It wasn't something subject to wear - it was an area of reduced thickness intended to sacrifice the lid and depressurize in a controlled way in the event the pressure vessel had been over-pressurized.