Replacing Hayward EC-65 pressure gauge

Stopped in to my local Mom & Pop pool store (whom I trust) for a new pressure gauge and was handed one that seemed right. Got home to find the one in my filter appears to be the same, but enclosed in a brown plastic case that then screws into the main filter housing. See items 28a, b and c at the following link:
http://www.poolcenter.com/parts_filt...flex_ec-65.htm

Had to head out of town right away so I didn't have the chance to go back for advice. It looks like the gauge itself is inside but I can't figure out how to open it. Any suggestions?