I'm not really sure - maybe Max knows?
I know we vary on an `optin` cookie - maybe that will provide some clues?

Christian's response (below) tells us the "mf-m" key in the X-Analytics header has the information we need here. So we're all set. "a" means alpha site and "b" means beta.

[From Christian]:

> For X-Analytics stuff,
>
> https://wikitech.wikimedia.org/wiki/X-Analytics

> would be the go-to place. I guess you're after the “mf-m” key
> there. That key is not set in varnish, but is set in the backend
> (MobileFrontendExtension), and it is still getting logged. But I am
> not aware of any reports still using that key.

Thanks Christian, sorry I didn't read the obvious documentation before dumping vague memories for awesome volunteers to help with.