That would probably be the hand that advances the cylinder.
In more modern Colt's the hand can be stretched ONE TIME to lengthen it.
This is done on the newer models with a 1/2" chisel with the face ground flat and very smooth and the edges and corners rounded slightly with a stone.
On the newer model Colt's the hand is stretched on the inside of the hand with the chisel in the slot near the bottom, NOT near the top.
I'm just not up on the New Army & Navy repairs, but if the hand is stretched to repair timing, the hand is put on a steel bench block on a FIRM bench and the chisel is positioned in the inside slot of the hand. The hammer is used to carefully peen the hand to stretch it slightly.
This is done ONLY if the hand is within range that stretching can be done, and with the understanding that hands DO crack and break during this.
After stretching the hand is carefully stoned to remove most of the marks left by the chisel.
Finally, take note that these guns were chambered for the .38 Colt Long, NOT the .38 Special. You can use .38 Special brass to hand load very low power loads, and for best accuracy, over-sized bullets to fit the larger bore of these guns.