The carriages that are now seen in the outdoor museum display of the original fabric of the U.S.S. Cairo are reproductions; there is one original carriage on display in the indoor museum.
U.S.S. CAIRO
VICKSBURG NATIONAL
MILITARY PARK
Vicksburg, Mississippi, U.S.A.
By Elizabeth Joyner
“CIVILWAR CANNON CARRIAGES REMOVED FROM U.S.S. CAIRO”
In an ongoing effort to preserve and protect the original
wooden naval gun carriages recovered in the 1960s
with the ironclad U.S.S. CAIRO, the gun carriages
have been removed from exhibit and replaced with
historically accurate reproduction carriages that will better
withstand the long, hot Mississippi summers.
One original carriage will be placed inside of the adjacent
CAIRO Museum, offering visitors a glimpse of an original.
Other carriages have been placed in a more suitable climate
that will be more conducive to their preservation and
protection.
The U.S.S. CAIRO was one of seven ironclad gunboats
named for cities along the upper Mississippi and Ohio
rivers during the American Civil War (1861-1865). These
formidable vessels were designed and built specifically for
river navigation. Their objective was to regain control of
the Mississippi River for the Union.
Commissioned in January 1862, CAIRO saw only limited
action until 12 December 1862, when the CAIRO traveled
with a small flotilla up the Yazoo River to clear the channel
of torpedoes (mines). At a point seven miles north of
Vicksburg, Mississippi, CAIRO was rocked by two
explosions in quick succession that tore huge holes in her
hull. The gunboat sank in 12 minutes with no loss of life.
CAIRO became the first vessel in history to be sunk by an
electrically-detonated mine.
Vicksburg National Military Park personnel worked in
conjunction with Historic Preservation Training Center
personnel from Frederick, Maryland, U.S.A. The crew
from the H.P.T.C. worked on reproducing the original
carriages for over a year before any on-site work was
begun. H.P.T.C. Exhibit Specialist Dean Wigfield stated
that all reproductions were constructed from the same types
of materials as were used to manufacture the originals and
a Pennsylvania blacksmith was contracted to re-create the
hardware.
Department of Ordnance Plans dating to 1851 were
obtained for use during the construction phase but proved
to be imprecise. Architects also found precise
measurements impossible to make with the cannon tubes
still in place. The first carriage put in place had to be
tweaked slightly on the spot, but ended up fitting fine
according to Wigfield. “The heaviest of the cannon weighs
8,200 pounds,” Wigfield said.
The carriages had been exhibited outdoors on board
CAIRO since 1984. Although the vessel is exhibited
beneath a canopy, time and the elements had begun to take
their toll on these unique carriages. As a segment of this
project, all original carriages received basic conservation
care to aid in their long term preservation. The carriage
removal phase of the project began 27 February 27 and
continued through 9 March 2006.
The following document relates a sad tale that goes a long way in proving the veracity of the old adage: "The road to hell is paved with good intentions."
It's on microfilm, so the qualities not the best, but if you scroll to near the end (gun carriages and tubes), there are drawings of carriages that show the cap-square key chain's placement on the cheek.
http://www.nps.gov/history/history/online_books/vick/cairo_hsr.pdf 
