Hal Finney's explanation of secp256k1 "efficiently computable endomorphism" parameters used secp256k1 libraries, archived from source.
The same optimization could be applied to any Koblitz curve (e.g. Short Weistrass curve with a=0).
I implemented an optimized ECDSA verify for the secp256k1 curve, based on pages 125-129 of the Guide to Elliptic Curve Cryptography, by Hankerson, Menezes and Vanstone. I own the book but I also found a PDF on a Russian site which is more convenient.
secp256k1 uses the following prime for its x and y coordinates: