The underside of the strap on the LP is the same leather as on top. It has a grippy or nappy feel, not slick and slippery on that leather. The 2nd strap has a soft black leather on the bottom, and the last strap pictured was ordered with sheepskin.
Here's what the ends of a 'cut to size' strap look like, and how I make one of these into an adjustable strap. I have always used buckles to make straps adjustable because I never cared for the 'weave through the slots' kind of design because they always seem to bunch up