If I remember correctly, a few years back I read about several mint employees that were producing clusters, sneaking them out and selling them for big bucks to error collectors. They got hit with large fines and some got more than a few years in jail.
As for how a cluster could make it out of the mint, your guess is probably better than mine. I can see 3 or 4 bonded coins slipping through once in a great while, but I don't see how a large lump of 15, 31 or more coins could get through all the sorting equipment and QC checks. I guess that's why they are so rare. It seems to me that they must have someone helping get these "errors" out of the mint.
I did find this one listed in a CoinWeek article
http://www.coinweek.com/featured-ne...mint-errors/ It's about 3/4 down the page and they explain how a cluster is made. Even though they say of this cluster "This is not one of the better ones.", it sold for $11,925, and once again, I'd love it in my collection.
I'd love to see pictures of clusters other than Cents, but I haven't found any.
As far as this being graded when it's only part of the cluster, I would think the TPGs look at it like any coin that has split or had another coin in the striking chamber at the same time. They don't need both coins or parts of the coin to grade them. Somewhere every coin with an indent, brockage, saddle strike has at least one coin that's a mate, and if you do have the mate, both get graded and both don't get the same grade in some cases.
It seems funny that with all the coins in this cluster that they couldn't find a date anywhere. Perhaps the other part got the date or dates.
I'd love to have this error, partial or not. It's fascinating to me and something that is so rare that I've only seen a few photos of them over the 40 years I've collected errors.
I love errors and the more impressive the error the more I

. I'm sure to some collectors that errors are just damaged coins, but I see them as accidental artwork.
If anyone can point me to some photos of other clusters, please let me know the address.
Ben