Think it's just a soldier's way of belittling the Army system. A Lance Corporal became "Lance Jack" because, when the rank was promulgated, the term "Jack" meant, sort of - everyman. The term "screw" in those days meant earnings, so a full Corporal - earning much more than a Lance Corporal - would be on a "full screw". However, this is only my personal interpretation of these meanings. If anyone has a more solid etymological base for a reply, please feel free to enlighten us. Cheers.