It's been a long time since I looked, but I think yours is "right". To the best of my knowledge, the original, spring-wound slings were always leather. I have seen a few "jury-rigged" web slings, but they did not roll-up like the original.
PS: Knowing your interest in Vietnam bringbacks, can I assume this is a new addition to your already exceptional collection?
Both the leather and the canvas slings are correct.
The thinner than standard sling of the CR39 was not a strong as regular slings, and weathered badly in hot humid surroundings the part always wound around the spool would rot quickly in such conditions, that is why the late or refurbished rifles had the canvas sling.