Strictly speaking of course, the buck 309 is a two thick or double spring knife. They do a good job of keeping it thin mind, around the width of a CASE Pen. Yellow delrin handles are slimmer than the very grippy sawn delrin.
Another thin knife that's inexpensive and really pleasant to carry is the CASE Slimline Trapper in yellow delrin, yes it's a load longer than the knives cited but it's still very pocket friendly- one of my faves anyway.
Thanks, Will