Hi Bret,
I've wondered about this too and the only thing I can come up with is that whoever did the ordering had their own whim as to SVRy, SVRR... etc.
Examples; Conductors reports from 1925, Sumpter Valley Railway Company. Ticket orders to Rand McNally, SVRR, 1905. Locomotive inspection report, SVR, 1916.
Most everything that I've seen so far as company letterhead is Sumpter Valley Railway Company, so this is probably what the higher ups prefered. Hope this helps. Does that collector have any other SV stuff?