The 348 and 409 engines have cylinder heads that don't have a combustion chamber, kinda like a diesel. The chamber is the top part of the cylinder. The way it's designed, it has to have a valve relief in the block.
Here's Edelbrock's version. The stock one is similar.
View attachment 1716342374
Here's a shot of the chambers in the block. You can see the reliefs for the exhaust valves.
View attachment 1716342375