Did they tell you they didn't have any in stock or was it just an oversight? If they told you they didn't have any in stock, just go back again later, if they still have none in stock, just ask for an older style field cap. If any of your instructors on your course have a problem with your hat, they'll get you a new hat while on course... that being said, you can only bring what you're issued, so they shouldn't have a problem with it.