In an attempt to resolve its unstable rear end, Ferrari introduced a new rear wing assembly in the Spanish Grand Prix, following comparison tests during Friday's FP1 with the older twin pillar layout. On Saturday, both cars featured the displayed single pillar rear wing, a design including a curve around the exhaust pipe and a single, central pillar to provide support to the rear wing.
Ferrari previously ran two pillars with fairly long chords connecting to the bottom of the rear wing base plane. In a straight line with a head on wing there isn't much of an issue, but with crosswinds or when the car is in yaw, the pillars were blocking airflow, a particularly unwanted effect on the low pressure side of the wing as it creates turbulence where the pillars meet the horizontal plane and hence also reduced downforce. Instead, the new design has a small swan neck attachment on the upper side of the rear wing and a much smaller chord, similar to Red Bull's single pillar mounting.
Along with this change, the team also had new rear wing endplates with curved louvres that provide a seamless attachment to the movable flap of the rear wing. It's a tiny change, but one that could help the flow reattach to the rear wing once the DRS flap is closed again. It's not clear whether that was or is still an issue with the F14 T, but it certainly has been at several teams in the past.
Following the Spanish Grand Prix, both drivers noted there was a notable improvement on the car, but with both struggling with oversteer, tyre wear and traction, the problems with the car are still far from over.