I've solved the problem, which has to do with the generating function for 'k'$k$.
For example:
- k = -108t^2 + 36t - 67$k = -108*t^2 + 36*t - 67$ always has even parity
- k = -108t^2 + 36t - 7$k = -108*t^2 + 36*t - 7$ always has odd parity
- k = -108t^2 + 36t - 84$k = -108*t^2 + 36*t - 84$ is 50/50$50/50$ odd and even
These, and other similar functions, generate nice groupings in residue classes modulo 504 and explain the population distribution of odd and even ranks in those residue classes.