Owen, the short answer is "Sights". But I bet you already knew that. Hope we can help a little bit with the following pics.
Almost all seacoast guns that we have studied have breech and rimbase or trunnion ring sights. I have seen blakely 20 pdr. field guns with graduated rear sights with a simple "V" notch and a blade front sight. We never have seen a seacoast Blakely with sights, but the two pics might give you some ideas. Love your gun and carriage! Very nice execution on the Chassis too.
Tracy and Mike
The “Canon de 30 par Voruz” 1864
The last major contract fulfilled by Captain Blakely for the Confederate States was that for the armament of four corvettes, CSS Georgia, Louisiana, Mississippi and Texas, ordered by Commander J D Bulloch, CSN, in France.
Forty-eight 16 cm cannon were ordered with 200 rounds of shot and shell per piece of J Voruz, Nantes, France; founders and engine makers. Unlike all of his other dealings with the south the details of the financial arrangements for this contract exist.
The correspondence quoted below is from Washington’s Official Records; the translation from the French is theirs. The repeated reference to “30 pounders” in these records is a mistranslation of “canon de 30”, actually a piece firing a 30 kilogram (68 pound) cylindrical shot. The forty-eight Voruz guns were of 6¾ inches calibre and each weighed 6,400 pounds.
“Canon rayé de 30” - Blakely Voruz 1864
A reconstruction from a casting by the volunteers of the Brigada Naval,
exhibited at the Escuela Naval, Callao, Peru
All of this from www.captainblakely.org/TheGuns.aspx You will notice that these naval guns have breech and rimbase sights. Larger seacoast guns are likely to have them too. 
This is a close up of the 100 pounder Parrott rear sight that we made for our 1/6 scale Parrott seacoast rifle. Having windage and elevation adjustments made sense when you had a stable seacoast mount, less so with a naval application. The Blakely sight would be very similar to these, while not exactly the same. We have an aperature rear eyepiece; the Blakely is likely an open notch.
