Antique Brass Sliding Bar Tri-Glide available in different sizes for webbing and strapping.  

Metal rectangle with adjustable middle bar for adjusting the size of webbing used in belts, bags, etc.

Metal rectangle with adjustable middle bar for adjusting the size of webbing used in belts, bags, etc. Heavier gauge, with wide opening. Comes in various sizes fitting 1/2" to 2"...

Black oxide electroplate webbing / strapping adjuster buckle.