Something very important about the large-frame Ruger single actions.
Ruger used to make the cylinders with six bit/reamer sets all going at once - in other words they did all six chambers at the same time. In 2004 when the mid-frames came out, they switched to a one-cylinder-bore-at-a-time process. This newer setup works better - overall dimensional accuracy of the cylinder is better and the variances we used to see between chambers is almost completely gone.
Ruger later moved this process to the large-frame guns like what you want. The way to ID a large-frame made with the new process is to look for the "lawyer's warning label billboard" (the "read the manual" bullpucky) on the underside of the barrel - that means new cylinder process. Side of barrel means old. Completely missing - predates the billboard completely, old process...unless somebody ground it off and refinished the gun. That isn't happening so much lately because the new under-barrel warning label isn't nearly as obnoxious. The changeover happened sometime in 2007.
There an exception of course
. In 2006 Ruger made a "50th Anniversary 44Magnum Blackhawk" that was NOT marked "Super". This gun was...weird, but nice. Small grip frame same as a New Vaquero, and was the only large-frame made with the "clicker" cylinder alignment that puts the chambers right under the loading gate on each click. It has a side-barrel warning label but the new-style cylinder. A real small-batch oddity, but a nice platform for customization if you want a 44Mag.
So anyways...you want a large-frame convertible 45ACP/45LC with an under-barrel warning label. That will be one of the most accurate box-stock Rugers ever made.
If it's not clear yet: ALL of the mid-frame series have the new cylinder type. There's also been some special runs of convertible 45ACP/45LC built on the mid-frame. You can spot these because they'll be "flattop" frames instead the normal "rear sight hump" frame. The mid-frame variants will be limited in 45LC to about 20,000psi loads, meaning basically 250gr hardcast at 900-1,000fps, 200gr JHPs at 1,100ish. And 45ACP+P would be...iffy. The large frame series is something you'd likely be more interested in - same accuracy if it's an under-barrel warning gun, and able to eat 45ACP+P all day, 45LC+P that beat most 44Magnum ammo in bullet energy levels while operating at around 33,000psi.
I'd go with the large frame series, myself. Weight isn't that much more. The gains in 45LC (and even 45ACP) are huge.