Yes, the adjustment is on the underside of the leather. Usually a little T shape that can go in various slots which are roughly the same distance apart as holes in leathers
I put Wintec Webbers on the dressage saddle (isabel werth) that my daughter uses. She had previously used normal leather ones with a buckle but find the webbers much better. Although I must admit if it were a leather saddle I would go for a similar style in leather (I think thats the snob in me)
I think they are great value for money though and would them again if needed.
I use synthetic leathers all the time except for a showing class. They are very difficult to spot as not being leather and I use them as I know they won't stretch or break. I have brown ones for my GP and black ones for my dressage saddle. But they are just a copy of normal leathers with the buckle at the top.