Although these are described as regular die trial pieces in the standard references, it is more likely that the off-metal 'trials' of this year were deliberately struck for sale to collectors.
In nickel as illustrated, only 2 or 3 are known all from regular business dies and thus there is a possibility these are actually mint errors struck on shield nickel planchets. The 1890 Parmelee coin was called Unc, whereas the 1891 Doughty and 1892 Woodside coin was called Proof. It is unclear if these 3 sales represent 1 or 2 coins.
These were also struck in copper J596/P660 from proof dies with a half dozen known and silver J598/P662 with at least 2 known, these from regular business dies.