I remember reading about training a small group of US Marines in parachuting early in WW II. It was discontinued for a number of reasons, very likely your father was one of that small group? I was taught glass cutting by an uncle long ago, (Thanks Riley), have cut down a lot of window glass, and broken some. A necessary thing when living in a house whose windows are no longer considered a standard size. Something I do while cutting is to do it on the flattest surface available.