It's a German studio project, featuring ex-members of beat groups The Rattles and The German Bonds (who would eventually form Lucifer's Friend) seems fairly likely to be the correct one, since those very same folks cut another similar exploito-psych album under the "band" name Bokaj Retsiem around the very same time. That album was simply titled Psychedelic Underground, so maybe they thought they did an even better job as Hell Preachers, Inc.!
hope that helps....