I've seen them too, and usually on the older
NGC slabs when these first started rolling out. I think these were a sales tactic by an early distributor applied to the less corroded pieces. It doesn't correlate to the 1-70 scale. I wouldn't be talked into a high price because its there.
With shipwreck coins, eye appeal matters and to each their own. I've seen this sticker on plenty of miscolored pieces I thought were ugly that happened to have less corrosion. I own a few heavily corroded El Cazador that I got cheap cause I thought they still looked cool.
My hoard of '82s is up to 241! 218 BC x 1, 118 BC x 3, 18 BC x 1, 82 x 1, 182 x 1, 282 x 2, 382 x 1, 582 x 2, 682 x 1, 782 x 2, 882 x 1, 982 x 4, 1082 x 1 1182 x 8, 1282 x 2, 1382 x 1, 1482 x 6, 1582 x 13, 1682 x 17, 1782 x 60, 1882 x 68, 1982 x 45