Perhaps they tried to sell it to the US (seeing the planchets have the same composition), but they gave up trying after five years and decided that they're just going to use it. This was six years before ARP, but it might just be because they know they'll ARP them and get more seignorage from people keeping the cupronickel coins instead of remelting them and selling for scrap. A bit far-fetched but not unbelievable, as the Mint aims for profit.
By the way, I replied to your PM.