I have a few Ruger # 1 Quarter Ribs for various configurations for sale. Hard to find and sell for $75 at Numrich arms. I am selling for $40 plus shipping.
To all that may ask the same question. This is the rib that allows the user to attach scope rings on the Ruger NO.1a, 1b, 1h, 1s, 1ab, etc. Here is a link to the Ruger site showing the exploded diagram in PDF format. The part number is "S-135" and it is shown in detail on page 34 of the 40 page instruction manual. I hope this helps.