In the original Ruger manual, that part was numbered MR-35 (MR-35W with white outline on rear blade). You could contact Ruger and ask if they still have one somewhere. If not, they may be able to suggest a source.
Sturm, Ruger & Co., Inc.
Customer Service Department
411 Sunapee Street
Newport, NH 03773
(Monday through Friday 8:00am - 5:00pm EST)
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