The central section boat tail has gone back to having a more well defined angular channel, similar to last year but not as pronounced. Gone is the flatter, more rounded solution from late 2017, and all of 2018 to date. They have also included the winglet on the outer edge, first seen in FP1 in Melbourne.
The new profile reduces the AOA of the central section, likely improving the stability of the flow, with a slight reduction in total expansion. I assume they are trading a small amount of total downforce for more consistent/stable downforce by keeping the central flow attached over a broader working range.
Hopefully this will increase the drivers confidence in the car and allow Seb to push harder.