Your 1971 came from the factory with a drum front brake and round sliders. Sounds like somebody installed the oval disk side slider and never bothered changing the other side. The theory was the oval sliders deal better with the increased bending force of the disk brake. Which is kind of funny, given that the disk brake wasn't much better than a well set up drum brake. For all practical purposes, what you currently have is probably fine. Most people won't notice the difference, and it should hold up unless you have a smaller master cylinder and try to do wheel stands. The round sliders have not available for some time, but Andover is now offering both round and oval.
Stephen Hill