Pardon the lateness of this reply. Yes, your .69-caliber 3-groove Minie with a plug-cavity base is "uncommon," because the majority have a conical cavity. However, many plug-cavity ones have been found at 1861 and early-1862 battlesites here in Virginia. I found a few "leftovers" on the December 1862 Fredericksburg battlefield. Can't quite call these "rare" -- but they are indeed "uncommon."
Regards,
Pete