Like a walk in the park on a Spring morning..
Lovely fresh grass green colour with subtle surface colour variations, characteristic of this kind of vintage casein plastic - each buckle is a little different.
4.5cm tall, and 4cm wide, gently curved for a perfect fit on your belt.
The inside aperture is just over 4cm tall.
I found these in an old haberdashery warehouse -they are in excellent unused condition. 
I also have a couple with prongs.