OK, I think I found the three Dahlgrens on page 243 of Olmstead, Stark, and Tucker's The Big Guns. All three are in fact IX-inch, all cast at Fort Pitt Foundry in 1863. They are numbers 678 (9,210 lbs, foundry no. 1475), 687 (9,280 lbs, foundry no. 1492), and 688 (9,300 lbs, foundry no. 1484). They are marked as "from Sackets Harbor," with no indication that they were ever mounted on a vessel. (There are other IX-inchers in Michigan that served on Hartford, though-- one in Cheboygan, one in Petoskey, and one in Gaylord.)
(The other big gun a few spots later in the thread is a Rodman Columbiad, by the look of the breech.)