Another collector answered the question about when the nylon buckles were introduced into service. He quotes Infantry Liaison Letter No. 9 dated 21 Feb 1958 which discusses the replacement of the metal, and the reasons why. It would certainly explain the reason why the metal buckled slings are harder to come by; they were replaced very early in the C1 rifles existence.
Credit for the following excerpts goes to Dan Martel.
https://www.milsurps.com/images/impo...lingPt1A-1.jpg
https://www.milsurps.com/images/impo...lingPt2A-1.jpg