Need help

[davewest]

Can anyone identify the whole in this photo, and what is supposed to go there? My son found it changing his oil, but the hole appears to be part of the transmission. Any help is appreciated.

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Edit. this pic is not my transmission but it gives a better look at the mystery hole. Any ideas?

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[GMCTRUCKS]

I'm thinking it might be a inspection Hole or to make a adjustment now I'm just taking a guess maybe look up a picture of another one just like yours and if it's the same let it ride.