Hmmm,
The main picture on
the Alpkit page does suggest that it comes with a single strap, for attaching to the saddle rails.
Mine may have been supplied with this (but I had it a few years and my memory is become rather suspect) and I may have removed it, due to using the bag in my Wildcat harness.
Some of the pictures further down the page - particularly those which show it mounted to the bike - suggest that two straps may be supplied.
Other picture don't hint at a second strap, and show the bag closed in the way that I used to close mine.
The strap (or straps) may be hiding somewhere, but I can't help thinking that I'd have used the supplied ones in the same way as the one that I've since made.
I'm wondering if these have been added to the package since I got mine...