It is a Remington cylinder and one of two different things have been done to the pistol that uses it.
1. The material just under the barrel threads has been machined away to allow the small tubular piece to extend forward to meet the newly machined surface between the barrel and the loading ram.
2. The forward area of the cylinder has been machined away leaving the small diameter to extend forward to the unmodified frame.
This would require a special barrel which has threads or a diameter that equals the minor diameter of the threads extending rearward to meet the face of the cylinder.
Based on my eyeball and my Remington Army I would say the second method was used.
This same concept was used by Rodgers & Spencer with their pistol but they chose to only extend the front of the cylinder forward of the cylinder face by about 1/8".
The end result of any of these ideas is that the blast of hot powder flame and subsequent fouling on the cylinder pin ends up on the small protruding diameter rather than on the cylinder pin.