Hmmmm...very observant! I don't remember ever noticing the DUpont numbers.

My first thought was that DUpont might be a different area...like North Las Vegas, because the addresses that I saw were on North Main Street.

My second thought is that they ran out of DUdley numbers, so added DUpont...but since they would dial the same, that wouldn't make sense.

So...back to the first thought, I pulled out the 1956 phone book to see if anything jumped out at me. It didn't...but eventually I realized that in looking at the DUpont numbers, I was looking at the same street names over and over. Here's some of the street names associated with the DUpont numbers...

North Main
Meissner
Hunter
Cassady
Baer
Wright
McCarran
Kinly (?abbreviated Kinley?)
Larnr
Spring
Jones

BINGO!! I googled Cassady and got this image of Nellis AFB...I think the mystery is solved