Brian...
My guess and I'd put money on it - is that this unit screws in... no visible screws on the outside dia.
I have seen a flanged design w/ a cover that hid the screws but from the photo it doesn't look like a removable cover.
I'm also guessing the raised center section provides surfaces to assist unscrewing by using an adjustable wrench.
Is there any brand indicated on the gauge that might provide a clue to the manufacturer?