There were no 20mm's found on Exeter wreck.
The images of mine that Dick linked to in his post turned out not to be a 20mm on the seabed.
Even if the gun itself (and shield even) had fallen out / off, the solid 'cone-shaped' support base (as it were) would have been expected to still be in either of the tubs - as it was a bolted down fixture - not something that would 'fall out'.
On closer inspection on subsequent dives a year or so after the images Dick linked to were taken, it was positively confirmed that the tubs were completely empty. All that remained were the round recessed 'slots' in the base of the tub (turret top as it were) where the various mounts shown in the image below (taken in Indian Ocean late 41 / early 42) were placed / fitted in.
Unfortunately, as beautiful as John Haynes model is, and it was beautiful enough that I wanted to buy it - but someone beat me to it - it is wrong in that respect. No offense meant John, it's still a BEAUTIFUL model!