Mine is similar and uses 2 O-rings. One is a snug fit around the threads, it makes contact with the filter housing where it fits through the hole and is secured with the large nylon wing nut.

The other is about the diameter of the gauge housing and makes contact between the gauge and the filter housing. Note, the fitting on this gauge is on the rear of the gauge and the gauge mounts flush/recessed to the top of the filter housing.