Brain-Teaser 483

From The Sunday Times, 30th August 1970 [link]

Only seven men still had ammunition. Aden had 1 round, Bill 2 rounds, Cuff 3, Dudd 4, Edge 5, Ford 6 and Good 7. The commander ordered there should always be five of the seven men on duty and that the duty roster should be so arranged that those on duty never had fewer than 17 rounds of ammunition.

Next day he was told that only four men were on duty. It appeared that three of the five who should have been present were sick and that the four doing duty included two who should have been off. It so happened that the four had between them the same number of rounds as the five would have had.

The commander had no copy of the duty roster, but he knew how many rounds each of the seven men had.

He asked how many rounds the four men doing duty had between them. When given the figure the commander said, after some thought, that he could say with certainty who the three absentees were.

Can you name them?