Unexpectedly high initial sales of the Proof 2016-W Mark Twain commemorative gold $5 half eagle by the U.S. Mint  pushed the coin into back-order status the morning of Jan. 15.

"Back order" means, according to the Mint, "This item is available to be ordered now, but it is not currently in stock. Additional inventory is being made. Please add the item to your cart to see when additional inventory is expected to be available."

The Proof and Uncirculated versions of the Mark Twain gold $5 coin went on sale at noon Eastern Time Jan. 14, priced at $364 for the Proof version and $359 for the Uncirculated coin. The prices remained the same on Jan. 21.

As of Jan. 21, the Mint recorded sales of 6,889 of the Proof coins and 3,284 of the Uncirculated pieces.

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The enabling legislation, Public Law 112-201, calls for the manufacture and release of no more than 100,000 of the gold coins.

U.S. Mint officials said the back-order status of the Proof gold coin has nothing to do with the Mint's problems with the certificate of authenticity for the Proof and Uncirculated 2016-P Mark Twain silver dollars.

The scheduled Jan. 14 release of the silver dollars was postponed after the certificate of authenticity was discovered to have a wrong citation for one of Twain's works on which the reverse design is based. The original certificates of authenticity must be manually removed from any finished packaging and new certificates printed for manual replacement in the packaging.

No new release date is yet announced for the Mark Twain silver dollars, which have a maximum mintage of 350,000 coins.