Is it not the case that your first and sixth pictures above show the twin-gun “angular” FN.54 installation? (but from the side, so the second barrel is obscured) All the other pictures presumably show the “earlier” blister mounting?

For what it’s worth, one of the options on the ancient Airfix model kit of the Blenheim IV is an angular under-nose turret similar to that in your first picture, with twin guns. Is this the FN.54 again?

The last picture is definitely Blenheim V, which as I understand it had a totally redesigned nose with a totally new rear-facing gun mounting.

I’m not an expert on Blenheims or FN Turrets so I wasnt willing to make definitive statements about the first and sixth pictures but they did seem to me to be different to the other “blister” type installations and yes personally I did wonder if they were the angular FN54, however I was hoping the photos and debate might encourage more expertise and information to be provided.

It would seem from Air Ministry’s very detailed post that the Blenheim V also had the FN54 or FN54A, but with a large fairing forward of the turret itself, and from his excellent drawings and details from various manuals he has confirmed my suspicions that the single gun blister “under-defence gun” were an in house Bristol design, perhaps slightly different in their Blenheim and Beaufort implementations depending on the size/shape of the parachute escape hatch?

regards

Mark Pilkington