I just took a quick look at the spare bell housing I have. The hole in question is machined, and MAY be lined up with the pocket for the clutch throwout arm pivot ball (I didn't look that closely.) That would be how that pocket was machined in the bell housing. If someone REALLY wants to know, I could go inspect more closely.
What prompted me to go look was the talk about a rubber plug for the hole. Why would GM machine a hole only to plug it up again?
