RogerT wrote:
....

I went to another property I have today and it has the same type of fitting.
Here are two photos from that property:
....

No better guess than before--that clearly is a nipple out the back end
on this one. I'm having trouble envisioning it being a check valve but
won't say it's impossible...

The gas company is now going to come back out to the original property (the
3-unit) next week and switch the 3 existing meters over to meters that can
be read remotely (they call them ERT meters, or something like that). When
they show up, I'm going to ask them what the fitting is and what it is for.
I'll post what they tell me back here when I find out.

Sounds like a plan. Was going to say you'll find out what it is when
the plumber moves them but same goes for this route, too.

Will be interested to hear what the mystery part is finally determined
to be. I'll be particularly interested in whether that first one is
complete or the actual working piece is gone in lieu of the reducing
coupling...

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