Geoff,
I have two Viking 1 transmitters, the first I have owned for over 10 years and the second is one I repaired for a guy getting into AM and then he lost interest so I bought it from him. Here is the weird part, my original has a steel cabinet according to my trusty magnet but the second one does not. When I repaired the second one the then owner brought just the transmitter without the cabinet and I haven't been inside it again since I bought it so I hadn't paid attention to the cabinet until your posting.
At a glance the cabinets look identical but both transmitters are on shelves with other gear and given the weight I am not going to pull them out for a full 360 examination
But I think it is safe to say you do have a Viking 1 cabinet and someone would probably like to acquire it since it is an unusual cabinet. I wonder now if the original design intent was for an aluminum case and the Korean conflict buildup caused a change in plans or if there was another reason for the change.