What does the W-15 mean on this Std Half slab?

Does it make it scarcer? and do you think the coin is Unc? TIA'

Closeup of coin

Closeup of coin

DPOTD-3
'Emancipate yourselves from mental slavery'
CU #3245 B.N.A. #428
Don
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Comments
"...Wiley's expanded die study on the 1861-O's as W-15. It was the last variety struck by the Confederacy before they closed down the New Orleans Mint. It is also an interesting piece because it pairs the last rev die, only used by the Confederacy, with a resurrected Obv 1 that was originally used by the Union in January before the State of Louisiana took over the Mint."
See http://www.doubledimes.com for a free online reference for US twenty-cent pieces
<< <i>It is hard for me to think of shipwreck effect coins as UNC. Perhaps UNC details or something is more appropriate. The salt water ruins mint luster among other things. Cleaning and stabilizing the coin after it is recovered doesn't help much either. >>
PCGS graded and slabbed the $20 Gold Coins that were recovered from a shipwreck. Perhaps gold does not undergo environmental damage?
DPOTD-3
'Emancipate yourselves from mental slavery'
CU #3245 B.N.A. #428
Don