It isn't necessarily a sling for an SMLE, it could be for a No4, or even an SLR. These slings were in use for an awful long time. Sorry, but I can't help you with the date of manufacture.
Books published to date... 'Belfast Diaries: A Gunner In Northern Ireland'... 'A Tough Nut To Crack - Andersonstown'... 'An Accrington Pal: The Diaries of Pte Jack Smallshaw, September 1914 To March 1919'.
Looking at the weave and the ink stamps I think its Indian. The 'AA' stamp on the brass work is one I've seen on Indian kit as well.
It definitely looks Indian to me as well!
'Ca' was a major Indian maker during both wars, but I'm not sure if this stamp refers to that particular maker though as 1978 seems like a late date.