As noted above,, the Ruger factory website to do a records search was & is confusing to many folks. Especially newer folks,, or those who aren't as computer savvy as others.
But,, as Wayno has mentioned,, the RENE Guide is THE best source of looking up correct information. Chad,, (and family) has spent YEARS compiling & recording data that is often easier, better, & more detailed than the factory website look-up. We collectors owe him a HUGE debt.
But the best way to get verifiable info,, is to send for the letter. And how to do that has also been shared above.
I recently got a letter back from Ruger with a few details on a model,, that was more detailed than what Chad had. I immediately sent him an email,, to allow him to update his information. The letters can often clear up mis-information or help prove facts about certain things.
But,, back to the OP,, as I mentioned,, most likely someone has modified this Single-Six,, and it's now just a great shooter.