Then you and I understand a split waistband to be different.
Those are split waistbands to me, with inlays on either side of the seam to allow easy alteration. A single piece waistband has no inlays. When the top of the waistband is left separated...either in a V or just a slit...that it sometimes called adding spring to the waistband. You'll see that as typical on English/fishtail backs on SR...including Steed. The RTW version of the "V" split is a sad and pretty much meaningless vestige of that, since on a straight waist it has lost its purpose in anchoring the braces as it would in a fishtail.