mustangdriver wrote:
The turret experts here can tell you more. I always thought that the ball was the same but the structure that supports them were different as the B-24 turret has a support that is able to be extended and retracted into the aircraft. There is a scissor that can be seen on the support of the B-24.
I'm certainly no ball turret expert, but I'm trying to learn.

Here's my take on the matter from what I've learned so far, Taigh & Co please correct me where I'm in error.
The turret and it's traverse ring, as seen in the above photos, would appear to be the same sub-assembly for both the B-17 and B-24 installations, only the supporting connection to the aircraft varied (Fixed in the B-17, telescopic in the B-24).
To differenciate between the two installations, each had a seperate AAF identification: A-2 series for the B-17, A-13 series for the B-24.
The ball turret is in essence an upside-down version of the Sperry A-1 upper turret, with the obvious mechanical differences. Similarly, the K-4 gunsight in the ball turret is an upside-down version of the K-3 sight in the upper turret.
I've mentioned the developmental variations before, the turrets in the above photos seem to be what I'd consider as mid/late production: Lower front windows deleted, windows added to the upper outer side sections, circular plate between the "seat" and the front window. Without being able to see the back of the turret, I'd guess these have external ammunition feeds with the corresponding stepped panel, incorporating the oxygen connector.
The barrel aperture differences I've also mentioned before, but exactly how the variations correspond to the A-2A, A-2B, A-2C etc. (and presumably the same for the A-13 series) I'd love to know.
I believe there are manuals available that cover the subject, but sadly the vendors won't supply to the UK for whatever reason.
All the best,
Paul