Unifaces from the mint are usually made with properly prepared blank reverse dies. i.e there is a rim of sorts, even if no design. Trials from unfinished dies may also be made, but tend to be in lead or similar soft material as the die has not been hardened at this point.
A perfectly flat surface on the blank side is much easier to obtain by rubbing down than replicating some vestige of a rim which you would expect as whether uniface or double sided it always has to be struck in a collar to prevent spreading, and so you would expect to see flow into the gap between collar and blank die (which would never be practical to use if a perfect fit without any tolerance).