from Breezie's post on "What is it?" 2/3/10
It appears these buckles were used from 1864 up until WWII. The all brass buckles are the oldest and like I said before, the steel and brass were made later like the one you found here and the ones on eBay. Good research to expand the dateline. We had originally thought only up til the 30s but this could be a 30's piece of equipment.
Its possible that these buckles were used for many different uses but pants was the most common. Breezies buckle is most likely for pants. The buckles that are patent dated, are CW era