Hello Dave,
From the Ser No, I believe your Mk3 is a 6th Series rifle made around 1966. I happen to have a Webley catalogue for that year which quotes the supply of rifle slips as follows:
"Plastic cover with leather sling for standard Rifle (Cov11) SEE DEALER" (for price)
"Canvas cover with leather sling for Rifle fitted with aperture or telescopic sight (Cov 4) SEE DEALER" (for price).
There is no mention of colour or presence of ties to secure the ends.
Hope this helps.
Regards
Brian