The engine is a 5.7 l small block I'm wondering what this hole is and how to plug it it's confusing to me I know cars and I've never seen this before please help

 

Holy crap that don't look good. I have a 93 Silverado 5.7 2 wheel drive. Just above the filter on the block is a pipe plug that's pressurised . I used that for a remote additional small micron oil filter. Hopefully you've found out what that hole is, not a freeze plug for sure. Let me know.

 

I assume the motor is running fine?
What year?