The choices are back, disc, and solid front. Pressure Gauge Blow Out Protection is used to protect the gauge being blown up in pieces in case of high pressure leak happen from the failure of bourdon tube. When the pressure inside the pressure gauge case reach some set pressure, normally it will be 1 psig, the disc or the entire back case will break and release the over pressure.
As another optional or additional design, when we used a pressure gauge in the severe condition, there are solid front designs in the market. This solid front design is a pressure gauge that separates the front part (dial, lens etc) from the back part. Usually it is a solid wall casting that separates the front & back part of pressure gauge completely.
As default, we can specify the pressure gauge blow out protection as disc type protection. While, if the service is severe and dangerous (very high pressure or contain dangerous chemical) we can use blow out back protection complete with solid front design.