What sets this one apart is that it has a Double Die Obverse (DDO). I acquired this one from a noted expert on Trade dollar varieties. Although somewhat scarce, the 77-P is the most available of all the P-mint Trade dollars.
Ron Guth: The 1877 Double Die Obverse Trade Dollar is a distinctive variety caused when the die was hubbed twice, with each punch slightly out of rotation from the other. The doubling is most visible on the letters of LIBERTY, on the top of the cotton bale upon which Liberty sits, the right end of the scroll (particularly under TRUST), and on the final star. Surprisingly, this variety does not command much of a premium despite its scarcity. PCGS has graded only four examples as of March 2011, three of which are Mint State, and the finest of which is a single MS-63.