The square tube actually covers over a round tube that rides in a socket. It has the same articulation as the newer round tube models.
It also has a clamp that you can use to adjust the length of tube engagement and thereby tighten the mower belt. I have never had much luck doing that on mine: first, the adjusting clamp is hard to get to, and second, the adjustment never holds. As soon as you ride around a while, the front end pushes back to ride against the rear chassis.