I saw a clip from the film and noticed that too. On looking more closely it is not a Bristol Freighter dorsal fin…it’s a different shape and not blended in.

It looks like a temp addition possibly because of the way they were throwing it about they needed an increase to the fin area for safety???

That was certainly a Srs.21 without a dorsal…normally

G-AIFV was damaged in the filming but repaired and returned to Silver City..minus the dorsal.

Garry