The plastic buckles were around long before the 82 pattern webbing was introduced. The 1975 FN manual shows them. I posed this question to some of the more advanced webbing collectors here in Canadaicon, and they concur with the timelines I have given for the metal and plastic buckles as being patterned on the 50s and 60s webbing. The likely reason that the metal buckled version is relatively rare compared to the plastic is that they were being replaced fairly early in the FNs timeline. The metal buckled sling is shown in the 63 parts manual (as is the Lee Enfield pattern of sling).

Also, I have seen many of the metal and plastic buckled slings with blanco on them. Blanco was long gone in the CF by the 80s.

As an aside, the C2 bras (chest pouch for magazines) can also be found with either metal or plastic hardware. The metallic clad ones have are of a more common canvas material while the later ones have the rubberized canvas.