Ruger finished out their 580- prefix with 100,000 New Mini-14 Ranch rifles. So once that happened, they combined the Mini-30's and Mini-14 Ranches in the 581-.
There's really no reason to have them in the same prefix anyway.
Ruger did the same thing with their older style Mini-14's and mini-30's once the Mini-30 completed their 189- prefix.
And all it has done is add to the confusion of the Ruger employee since I have seen out-of-sequence guns produced right after the new series' of Mini-14's and Mini-30's came out...one in particular a 581- prefixed Mini-14 on Ruger's trade show display the same year the models were announced. Ruger caught a few of these and gave them an "R" prefix serial number before they were shipped out (for "RANCH").
Currently, Ruger has four different series' of Mark III pistol and each started with their own prefix...227-, 228-, 229- and 270-. Well, the 227- and 228- guns have filled their prefixes and joined up with the 22/45 Hunter in the 270- prefix. Another 30,000 or so 229- guns and they will also hook up with wherever the 270- prefix is at at the time.
Chet15