Luttwak writes a potential recruit for the conspiracy can be identified by tracking officer careers and the years they got to each rank. Those who lag behind their classmates in military school are probably resentful of the regime and will see a coup as an opportunity for a promotion, particularly if they lagged behind because of unfair treatment. Well, look at the ranking of cadets of the 1918-1921 class in their final exams:
>Kruel: 18th among Cavalry cadets (cavalry had more prestige than infantry)
>Mourão Filho: 8th among 98 Infantry cadets
>Costa e Silva: 3rd among 98 Infantry cadets
>Castelo Branco: 33rd among 98 Infantry cadets

This ranking is usually a good hint of who gets priority for promotion in the rest of their lives. Their career progressions were then:

>Kruel: colonel 1947, brigade general 1954, division general ~1959, army general 1963
>Mourão Filho: colonel 1948, brigade general 1956, division general 1962, army general 1964 (after the coup)
>Costa e Silva: colonel 1944, brigade general 1952, division general 1958, army general 1961
>Castelo Branco: colonel 1945, brigade general 1952, division general 1958, army general 1962, marshal 1964 (after the coup)

It's also worth noting that Mourão Filho's promotion to colonel was by time served, not merit, and despite having served in Italy which is a badge of prestige. He became a brigade general because the President was his friend and born in the same city. His promotion to army general happened at a time the ranks got refreshed after the failed 1961 coup and he had been one of the generals who opposed it.
Hence he fits into the profile of a resentful officer discriminated against in promotion. Why was this? Not out of incompetence as he was consistently praised in his earlier career. Rather, his authorship of the Cohen Plan made him the scapegoat for Vargas' coup d'état in 1937 and he was left isolated. Vargas' chief of staff, one of the key men behind the coup, in 1955 directly accused Mourão (a minor captain in 1937 and a random colonel in 1955) while absolving himself of any responsibility. Mourão took this to the military courts and managed to force the accusation to be retracted, but to this day he's remembered negatively for his role in both 1937 and 1964 and usually described in the most negative light possible.