You asked about the age of your buckle. First let me say I do not see myself as an expect on Civilian buckles (my area of study has been Military ones). But I notice your buckle's middle-bar isn't plain/smooth, it has tiny flanges on it which keep the buckle's tongue from sliding off-center on the bar. I have not seen that characteristic on any civil war era or earlier buckles.
Also, I haven't see "centering" flanges on any buckles which I'm sure are pre-1900. If somebody here knows of "centering" flanges on pre-1900 buckles, please speak up with examples. Until somebody does, I think your buckle is from sometime in the 20th-Century, or perhaps latter-1800s at the earliest.