Howdy
Excellent question about the location of the ratchet teeth.
No, the cylinder is all one piece, nothing has been added on.
I don't have an original 1860 Colt, but I have a couple of Pietta replicas.
Here is a photo of the Pietta Cylinder and the Richards Conversion cylinder. You are correct, the teeth are centered on the chambers in the 1860, they are between the chambers in the Richards Conversion.
A few of the cylinders for the Richards Conversion were reworked from original percussion cylinders, most were newly fabricated for the model. But they were all one piece, nothing was added.
The way they did it was the Conversion cylinder is not as long as the percussion cylinder. So the tips of the old teeth could be faced off and new teeth machined, in the new positions.
In addition, the hands were modified. The patent called for the old one finger hand of the original 1860 Colts to have an extension added that pivoted inward as the ratchet teeth rotated. The modified hand would push the teeth around the appropriate amount. Apparently this was never actually done. Instead, a two finger hand, not too different than the hand of the SAA, was fabricated to push the cylinder around the correct amount. You can see the hand next to the hammer in this photo.
