The black cap screws off and you can transfill it with a yoke adapter that came with it, or you can use a high pressure whip, or put a yoke block on the whip and use the adapter.

The first stage has an overpressure relief valve, a high pressure port with a button guage, and two low pressure ports.

Abyss marketed it, but I'm not sure who actually made it. My rebreather friends dissected it with interest and couldn't figure it out either.