The first chokepoint was on the shores of the Paraibuna river, in the moment with a lot of water. Here there were both rail and highway bridges, the former east-west and the latter north-south. On the northern bank, the Paraibuna rail station. Across an extremely narrow bridge, the village of Monte Serrat. The mighty Paraibuna Stone dominates the horizon. Then for a while the rail and highway proceed paralell to each other and the river, the latter on the southern bank, until the town of Comendador Levy Gasparian. The literature doesn't use urban names, only rail stations, and on the other side of the river is the station of Serraria. Note that none of the armies was actually using the railway. The bridge in the middle is part of the new highway which didn't exist then.

It was good defensive terrain, though so rocky it was hard to dig shelter. Dynamite was set on the bridge. It would've been easy to defend by detonating it and entrenching on the northern bank, but for the same reason the bridgehead was vital for offensive capacity.

By 18:00 the 1st Caçadores arrived to take the bridge - but it was already taken. They entrenched between the river and the Paraibuna stone. There was now a frontline, tiny as the bridgehead itself. It was not the entire battalion, Muricy calls it a company, there were perhaps 5 platoons on the frontline, and the (not very reliable) figure of strength is 300.
Kerensky was in person. Muricy arrived by 21:00 and Mourão Filho very late.