The one you're referring to is FS#021.8. There is also a second 1955DDO, FS#021.9, that shows the strongest doubling on the motto IN GOD WE TRUST, and less so on LIBERTY. And there's also the poor man's doubled die which I think is strike doubling, with some doubling showing on the last 5 in the date. As with any coin that carries a hefty premium, counterfeits exist for FS#021.8. The real deal has two faint die scratches on the left side of the T in CENT. They extend downward from the top horizontal bar on the left side.
There are actually about eight different doubled dies in 1955. They come on both obverses and reverses and I think at least one of them is on a Denver issue. But only the one well known strong one commands any serious money and it is a Philadelphia coin.
Comments
You have it right.
Registry 1909-1958 Proof Lincolns
Frank