It is my honest opinion that the object highlighted by the arrow in the clip below is a piece of JFK's skull, and not a rose. I've analysed this piece of film so many times before and was quite shocked when Steve Barber pointed the object out.
To me, it is an object rotating as it falls to the limo floor. It is triangular(ish) in shape, has flat sides and edges. It is a reddish-pink colour rather than the deep red of the roses.
John posted this clip, which appears to show a large piece of JFK's skull being blown towards the area we see the object pointed out by Steve Barber rotating and falling below the edge of the door.
The bottom line is this - if the object is indeed a piece of skull them it must have been found in the limo as we see it falling to the limo floor. And this is indeed the case, although as with nearly everything in this case, there is a certain amount of confusion about who found it, what it consistent is that a piece of skull, measuring approximately 10 x 6.5 cm was found on the floor in the back of the limo.
Paul Seaton deals thoroughly with this matter at
https://paulseaton.com/jfk/frags/bounce.htmThere can be very little doubt that the large piece of triangular(ish) shaped skull discovered in the back of the limo is what is being shown in the above clips from the Z-film.
As Seaton notes, both the triangular piece and the Harper fragment are blown forward. Yet another piece of confirmation that the head-shot came from behind.