SpaceX shared some views of Booster 9 with what they refer to as to allow hot staging, with two workers on the new hardware that add perspective on the size.
The hardware is both a heat shield that one worker is sitting on top of and the collar atop the already existing booster with lots of slots left open in it to allow the flames to exit the top and reduce heating of the methane tank, the top tank in Starship. Another worker is seen with his head and arm sticking through one of those slots just above the front left grid fin.
The interstage adapter looked like this as it was being lifted into place.
You can see that it's a lot of empty space. Both images from SpaceX and edited a bit by me to resize and brighten them a bit.
"Vented interstage and heat shield installed atop Booster 9. Starship and Super Heavy are being upgraded to use a separation method called hot-staging, where Starship's second stage engines will ignite to push the ship away from the booster," the company wrote today (Aug. 18) in a post on X (formerly Twitter) that shared two photos of the new hardware.
"The superhot plasma from the upper-stage engines has gotta go somewhere," Musk told journalist Ashlee Vance in a discussion on X on June 24, during which he revealed the design change. "So we're adding an extension to the booster that is almost all vents, essentially. So that allows the the upper-stage engine plume to go through the sort of vented extension of the booster and not just blow itself up."
This set of pictures makes me lean toward my belief that the road closures on Monday (early next week) are to move B9 back to the Orbital Launch Mount for more testing. Launch is still "out there" waiting on the FAA and perhaps the lawsuit, but this makes it look like B9 may be considered ready to roll back out to the OLM for follow-on testing.