Thanks for the info. Unfortunately it did not work
So I retrieved 2 more cRIOs from storage, one 4 slot and one 8 slot.
The 8 slot behaved identically to the first unit (also an 8 slot) - "Rebooting CompactRIO device..."
The 4 slot unit was found by macc address and reported an unknown image. It did have a slightly different team ID for testing. Formating it to 1310 DID work with all the expected update messages.
I do note that when running the Network console to watch the cRIO consoles, on the 8 slot devices I see a number of threads (multiple Command Server TCP connection threads and FTP Server Connection Thread) terminating. This doesn't sound good as FTP is the method through which things are downloaded.
More head scratching...