Redbob,
To address a few questions for you as I understand the guns;
1) All three guns on the Pee Dee were on pivot.
2) I have studied the Brooke guns for 15 years in conjunction with the war as it related to Selma. That takes in a lot of research when extended to Catesby ap R. Jones, John Mercer Brooke, Brooke tubes, naval artillery in general, etc. In that time I have never seen a single reference to a 7" Brooke rifle on a naval truck carriage. I would be quite interested in any references to that effect.
3) The VI.4" rifles were typically but not exclusively used in broadside. As such the usual carriage was the Marsilly as I understand it - I am NOT the carriage expert here, FWIW.
4) My guess would be that when the guns were scuttled the sailors doing the work probably dismounted the tubes - leaving the pivots to burn with the rest of the boat.
It would be great if someone would step in with better information on all of the above. Ted Gregg of Conway, SC (?) is well versed on the story & recovery. I would add Glenn Dutton and Rufus Perdue to that list as the most definitive in terms of the history of the boat and recovery efforts. They currently own the shipyard property. Hank Boyd of Chattanooga did substantial research on the Pee Dee/Mars Bluff shipyard at the time and may have more information in his notes if you choose to do more detailed research.
Again, FWIW.