For anyone that comes across this same problem, I lodged a ticket with Meraki, and whilst they looked at it, there was no fix coming, just saying it was an issue with Samsung. They also went with the possibility of the knox security bit being flagged (but like I said, brand new, straight from the factory devices, so I know they never had any other OS images on them) As luck would have it, these new A7's are bleeding edge, so over the last 3 weeks there have been 2 OTA updates, the first one didn't fix anything, but the second that arrived yesterday has resolved the issue and we can now register the devices. The only down side is you have to skip through the setup without using the MDM so you can run the software update on the tablet, then factory reset it so you can then put the device on the MDM, but at least it works.
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