Howdy
I have one exactly like it. Five and one half inch barrel and the rear sight is drift adjustable for windage. Mine was made in 1961.
It is easy to determine if yours was sent back to the factory for the safety modifications. Make sure the gun is empty and then cock the hammer. If you see a transfer bar rise up partially covering the firing pin, it has been modified. If not, it has not been modified.
Ruger will modify it for free. Personally, I recommend against it. Learn the proper way to load an old fashioned single action revolver. NEVER load all six chambers. Always load it with only five rounds and make sure the hammer is ALWAYS down on an empty chamber. If you lower the hammer on a live round, and the gun falls to the ground and the hammer strikes a hard surface, IT WILL FIRE.
The proper way to load a revolver like this is to
1. Set the hammer to half cock.
2. Open the loading gate.
3. Load one chamber.
4. Skip one chamber.
5. Load four more chambers.
Then, keeping your thumb on the cylinder so it does not roll backwards, close the loading gate and then bring the hammer to full cock. Then carefully lower the hammer. The empty chamber will be under the hammer. Unfortunately the chambers on these revolvers are counterbored so you cannot see if there is an empty chamber under the hammer. So practice this technique over and over again with snap caps so you can check if the chamber under the hammer is truly empty.
By the way, if you go to the Ruger website and click on the Customer Service tab, and then click on Instruction Manuals and Product History, then click on the choice for revolvers, you can bring up the product history of your revolver and determine what year yours was made. Ruger lists the first SN for each year.
Enjoy your new Single Six, and be careful how you load it.