"First Light" film query

Seeing as it’s Geoffrey Wellum’s birthday today, I thought I would watch this on DVD again.

In the first take-off sequence, there is a shot over the shoulder of the pilot as the Spitfire takes off. It has a cannon ammunition blister, but no barrel.

I know Spit XVI TE184 was used for a number of scenes, but it appears complete with cannon barrels throughout.

Also, while there is a wealth of recycled Battle of Britain footage used, I know that none of the Spits in that film had blisters on the wings.

Anyone know more?