Please believe me that I take absolutely no pleasure in having to tell you that it is definitely a 2nd-half-of-the-20th-century imitation. It cannot even be called a Reproduction (a copy), because no such buckle exactly like that one existed during the civil war. Your "Oval CS" buckle is constructed like the civil war Oval US buckle, but there was no exact Confederate equivalent. (There was such a thing as an Oval CS buckle, but it did not have the so-called "lead-filled" back, and its front was somewhat different too.) Thus, the type of imitation you found is known as a "Fantasy" item. This kind is still being manufactured in the 21st-Century and sold as a battlefield-park souvenir ...and unfortunately is frequently seen for sale on Ebay mis-advertized as a genuine civil war CS buckle.
Info-source: "Confederate Belt Buckles & Plates" by Steve E. Mullinax ...which is widely acclaimed in the civil war relic-collecting community as being the best reference book ever done about Confederate buckles and plates.