Howdy
You would be amazed how many shooters who are used to pistols with adjustable sights find that their fixed sight guns are shooting to the left, particularly if they are right handed. Take a look at one of your adjustable sight guns, and I'll bet you a donut the rear sight is adjusted slightly to the right.
This is absolutely typical. If your rear sight is adjusted to the right, it is compensating for the shooter's tendency to shoot to the left. For a righty, too much finger on the trigger will tend to push shots to the left.
Question: where are you placing the trigger. If you place it in the crease of the joint behind your knuckle, that is classic 'too much finger on the trigger'. Try pulling the trigger with just the pad of your trigger finger. This will allow you to pull straighter back without pushing the gun to the left. There is really only one way to pull a trigger. All your finger is capable of doing is curling. You cannot pull the tip straight back. All you can do is use the pad of your finger. This will allow you to pull straighter back with less tendency to push the pistol to the left than happens with the trigger in the joint.
P.S. If you want to find out where the pistol shoots, shoot it from a rest. Shooting it offhand, particularly one handed measures how well you can hold the gun steady, not where it actually shoots. If you want to find out where the gun shoots, remove the human element and shoot it from a rest. That is why Ransom Rests were invented, to remove the human element. Once you learn where the gun actually shoots, you can work on trigger technique to make it shoot where you want when shooting it offhand.
I shoot single action revolvers all the time in CAS. I know that I pull my shots to the left. I know it is not the guns, it is me. I compensate by aiming at the right half of the target, but I know darn well it is me, not the guns, that are shooting to the left.