Basically, there were two different materials - cotton web (most common), and much later (in the 50's), some olive nylon slings started to be seen, that could be used on both the L1, and the Enfields.
The first cotton web slings were khakhi (tan) with brass hardware, then into the latter stages of WWII cotton slings in olive were produced, as it became more common for Commonwealth troops to be fighting in temperate or jungle conditions. If yours has black metal, it could very well be a Canadian made sling, from late WWII.
Based on my interpretation of bits and pieces I have heard / read / seen in my decade of Lee Enfield interest... Someone else here may contribute more objective and detailed information.
Cheers,
Neal