a quarter to six
Both stood where they were . It was the men of mystery and tobacco that Dexter was after , they thought . But they were wrong . What had brought Dexter to the vault was the fact that he had seen two boys , who looked uncommonly like O'Hara and Moriarty , go down the steps of the vault at a quarter to six . He had been doing his usual after-lock-up prowl on the junior gravel , to intercept stragglers , and he had been a witness -- from a distance of fifty yards , in a very bad light -- of the descent into the vault . He had remained on the gravel ever since , in the hope of catching them as they came up ; but as they had not come up , he had determined to make the first move himself . He had not seen the six unknowns go down , for , the evening being chilly , he had paced up and down , and they had by a lucky accident chosen a moment when his back was turned .