Not sure. You say you bought it about five years ago. Dealer stock would still have the old rail and would also have started stocking the new rail. If you bought a packaged model number it would specify the correct rail as part of the package. And some packages had all of the instruction with the saw under that model number and nothing with the Unifence. Most had nothing with the saw and the manual/instructions with the fence.
Or it could be a simple mistake where the old rail was supplied by mistake by either the dealer (did it come packaged separately or in the fence box?) or by Delta (certainly would be the case if supplied in one box with the new instructions).
The 52" and the later 50" rails never came to the factory, went directly to the distribution center. I don't recall if the 30" were handled that way too. I know that some of them (50") were mislabeled by the supplier about the time PC took over the Delta distribution (just one of many problems at that time, in the beginning of 2000).
It's been a long time since you bought it so I'm not sure that there is anyone (with B & D) who could sort it out after the fact if you wanted to change rails to the new one in the event it was a mistake. As mentioned, nothing wrong with the old rail, just didn't have the flip stop feature and a little harder to assemble.
Frank