Joey has done a good job of much of the explanation of the conversion issue. But he's a little off on a few points.
As noted, Bill designed the OM single-actions to be better & stronger than the discontinued Colts etc. They were a big success.
But some folks lamented the fact it was called a six-shooter, but you should only load 5 for safety. A method that was decades old & accepted by KNOWLEDGEABLE people. You could load 6 if the need arose but for general carry, you didn't.
Well Bill came out with the New Model in 1973,, with a safety transfer bar to "fix" that issue.
A few years later,, an idiot shot himself because he had an Old Model, and didn't understand or know to not carry it that way. He sued Sturm, Ruger & Co. Up to that lawsuit,, Bill had never lost a lawsuit. In that case,, the judge ruled against Sturm, Ruger & Co with the comment; "They knew they had a defective design, and that was why they built the New Model." That judge disregarded history AND the fact that other makers still had similar actions as the OM Rugers.
That pissed off Bill and in about 1977,, he had the ugly "billboard warning" added to all his guns. That's a direct result of the lawyers & the lawsuit.
Then Bill also had his engineers design the "Transfer Safety Bar conversion" to be capable of retro-fitting all OM SA handguns. They offered the safety upgrade for free, and still do so. I'd have to go back in my records to see what year that was. Initially, when they received an OM in for any work or the upgrade,, it automatically gets the safety upgrade,, even if the customer didn't request it. But the OM parts are returned to the customer. And they would mark the frame, on the inside,, hidden by the grip frame with a circled "R" to show they had performed the safety upgrade. That was done to be capable of proving to any future jury or judge that they had done more than their due diligence for safety. Now,, I don't think they mark the frames anymore.
And if you have a gun that has had the upgrade,, and want to return it to "as shipped" with the OM action,, you can do so easily. When they built the upgrade parts, it was designed to where no modifications were done to the gun that affect the value or function.
So,,, in the case of the new member here,, he needs the OM parts, and Joey is offering help.