I was delighted this week to see first hand a friends copper alloy buckle frame with horse terminals. Typically dating to the 4th and 5th centuries AD.
Reverse is flat and plain. The front has a raised border around the frame, and the horses heads are plain.
Suspected to be a development of the earlier dolphin buckles and appear to be a british style, with very few found on the continent. A similiar example to this one can be found in Appels and Laycock "Roman Buckles and Military Fittings", page 209, fig. SL8,13.