Given the number of them around, I don't think they were confined to "Harris-period", but the current state of "knowledge" is that there was only one pot of cobalt ever melted, so all those bits containing cobalt would be contemporary with each other, and that was at some point when MH was still there.
Any further information about this "one pot of cobalt melted" has not been forthcoming, and I don't know if it has been in any way officially confirmed.