Always nice to find the pre WW1 examples, I have RIA 64145 with a excellent bore RIA 3-07 dated barrel and the early square sided slide like on Rays Rock Island.
A few years ago I did restore some nice ( 3576 & 3670) Rock Island receivers from the CMPauction, not worth much but nice looking. Finding pre WW1 Rock Island barrels is very difficult but I was able to find early SA barrels in decent condition for restoration.
Best sub caliber insert that I have used is the 32 ACP or 32 S&W Long, both are accurate.
A total burned-out bore is best with a quality 22 rim fire insert and M2 bolt with headspace
adjustment screw, very accurateAttachment 93849Attachment 93850Attachment 93851Attachment 93852Attachment 93853