As you know but others may not, the Spiller & Burr was a Confederate pistol that some say is a copy of the Whitney revolver, the main difference being its' brass frame.
There are other differences between the Whitney and the S&B as can be seen in this photo of Italian reproductions of both.
Notice that the S&B has increased the length of the front of the frame, probably to increase the thread length on the barrel.
That would be one way of adding strength to it to make up for the substitution of brass/bronze frame material.
Another difference is the clearance between the face of the cylinder and the frame.
Notice that there is almost no gap in this area on the S&B but the Whitney shows a large gap with just the rear of the barrel coming close to the cylinder face.
This feature is an accurate representation of the Spiller & Burr as it is shown in all of my Confederate pistol books on the existing original guns.
This small gap was one of the mistakes made by Remington on their first Remington-Beals Army and Navy pistols.
It caused the face of the cylinder and inside area of the front of the frame to rapidly build up fouling that locked up the gun. The Army was not impressed with this feature.
One of the first changes Remington made was to move the inside of the front of the frame away from the cylinder and extend the rear of the barrel so the barrel/cylinder gap would remain small. That allowed the sharp edges at the rear of the barrel to scrape away the fouling from the cylinder as the gun was cocked.
Notice that the Whitney in the photo is using the Remington "fix" for this problem.
I've always thought it interesting that the Confederate requirements were very specific. The pistols were to be reproductions of the 1851 Colt Navy.
As Spiller & Burr were already pretty much tooled up for the Whitney copy, apparently the Confederate's covered their eyes to the obvious differences between these guns and the open top Colt 1851 because they gave the go ahead with production.
Oddly, considering the South was hurting for money at the time, I've read that the Spiller & Burr pistols came with silver plating on the brass frame.