Rear Engine Fire Wall Rivets missing or broken

[Skymaster337B]

Attached is a pic of the rear engine fire wall, under the muffler, where I found rivets either broken or missing. The parts manual doesn't list any specific rivets for this assembly. Any ideas? The whole fire wall doesn't seem to be a load bearing structure. Has anyone ever experienced this before? Or even better yet, check your airplane and let me know what it looks like.