53 cracked block, plate question

Hello All

I bought a 53 special a while back and have been going through it recently and after scraping off what I thought was some RTV I found what appears to be a crack near the freeze plugs. It was filled in with some grey material, maybe some jb weld or similar. Aside from that though, what is this plate with a single nut under the repaired freeze plug? I want to clean up the area so I can repair the crack. Whatever the repair was before it later a long time. It was covered in dirt and grime as if it was there a while and didn't appear to leak when I ran the engine.

Thanks for the info.

Near the back of the engine? About 5 inches by 1 1/2 inch. The crankcase ventilation intake.

Ben

Id verify that the headgasket is bad or the block is cracked before doing anything else. Stupid question but Im assuming youve ruled out the thermostat by removing it?