Err...yeah. So, the answer actually is...MS but too low MS to be worth grading to whomever submitted it.
it was a "bulk" submission.. that is the only time NGC gives out letter grades other then details.. in this case the submiter probably got anything 64+ as numerical grades the rest letter grades... It is a discount deal they do for bulk submissions for dealers... In this case BU generaly means MS60-63
When you do a bulk submission...you can set the threshold for a number grade wherever you like. The truth is, these coins would be harder to sell with an MS60 or 61 grade on the slab anyway. But, people will buy them with a BU slab because to the uniformed collector it seems better.