The slings I have on my Ross's have rivets. One is 1916 Dated (made by Eatons in Toronto). The other is not marked at all but beleived to be an early Canadian made sling and is of lighter brown leather.
I have a WWII Brit made sling dated 1942 (homeguard) and it is sewn. Then again I have a leather frog (also homeguard) dated 1940 and it is held with rivets.
Whenever I see sewn vs rivets on other countries slings, rivets are not used on wartime slings usually because brass is required for ammo, so thread is a substitute. Not saying that is a rule, but it may explain the lack of rivets on later made slings.