This is the thing in question
Here is a bigger picture. Main natural gas line comes in, this thing is attached before the meter. A half inch natural gas line runs out of it towards the front of the house, where there used to be a gaslamp (still there, now electric).
I think I remember the gaslamps burned all the time because resources are forever, or at least they used to be. So my guess is pressure regulator or emergency cut off, or both???
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House was built in the 50s if it matters, though the meter may be a little newer.