It's a low pressure fuel injection pressure tester. Since the gauge only goes to 15 psi, it's meant for TBI or throttle body injection from like the 1980's early 90's pick-ups etc.
Don't confuse Grainger's list price with reality.
http://www.grainger.com/product/WESTWARD-Tester-1MZT4?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/1MZT4_AS01?$smthumb$