K423 and K128 are the Kiepert AntiCevian Mates of the Brocard (fourth) cubic K020 and also K422. See explanations in Table 32.
K423 and K128 are members of the Thomson-Grebe pencil. See Table 13.
K423 is a an isogonal pK. Its pivot is X(3329) = a^4+b^2c^2+2a^2(b^2+c^2) : : , the harmonic conjugate of X(385) with respect to G and K.
It is a cubic anharmonically equivalent to K020 as in Table 66.
Locus properties :
The 2nd Neuberg triangle and the anticevian triangle of M are perspective if and only if M lies on K423. The locus of the perspector is K422. See the related K128.