A correction to the admittance equation of the equivalent circuit for a symmetrical two-terminal-pair junction as proposed by Montgomery is presented. The circuit used by Montgomery is built with two sections of transmission lines separated by a shunt admittance. The mistake in the equation was detected while using that circuit in cascade to model a microwave bandpass filter. A procedure is established and implemented to identily the mistake in the equation. Next, the correct equation is developed using the n equivalent circuit. Samples of double posts in the P-band (two-terminal-pairs junctions) are built and characterised. The experimental S-parameters are applied in the equation proposed by Montgomery and in a procedure by the authors, showing excellent agreement with the latter and a constant error in Montgomery's.