Some additional clues:
TCP Global, Ditzler DL-31557, 1957 Interior
Dupont History Site, DUCO 73605, 1957 Interior Chip
Dupont History Site, DUCO 73605, 1959 Interior Chip
Dupont History Site, 1956 Truck Interior Text
Dupont History Site, 1957 Truck Interior Text
Dupont History Site, 1958 Truck Interior Text
Dupont History Site, 1959 Truck Interior Text
DUCO 73605 was said to cross over to Dupont DS174 - but one onwer tried it and said it was too light Dupont DS174

Have send request to TCP Global for their insight to this question. Will post answer if I get one.
Thanks, Joe