It has never been clear to me whether that is a single "market on close" trade or a composite of trades during the day that happen to get "printed" after the close. Although the post-market print is usually a large number, this one today is obviously excessive in size. Given the size, it most likely involves a liquidation by an institution, but whether the buyer is covering a short is unknown.

When the 12/31/12 official short report is issued, it may shed some light on the counter-party buyer, especially if the short figure is reduced by a large number as such. Until then, I would not make too much of the composite transaction other than perhaps concluding optimisitically, it is a transfer from "weak hands" into "strong hands," assuming it is not faciliated to cover a short.

As for some of the additional post-market trades around the price of 4.58-4.59 (below the $4.66 close), I would not read anything into those declines other than to assume that the buyers took advantage of the small sellers at illiquid moments.

Silverbull50