If you look inside the clasp where the slide adjuster sits, the strap is held in there by a spring bar. You need to insert a forked spring bar tool between the rubber strap and the clasp and then compress the rubber to reveal the spring bar. It's a bit tricky to get the first time but not too difficult. I use a small screwdriver to compress the rubber so that I can then get the spring bar tool in.
And of course welcome to the forum, post a pic of your new acquisition for us to drool over........
