Recovered one of these myself just this week (5/11 or so 2021). Bit tarnished but stunningly unbent, smooth and with unchipped rims. Ran into a lot of flatware over years and this was in best shape of them all; impressed. But yes, little to NO info on Union Nickel Co.; x the online maker's mark catalog that may not have been seen on original post IDs with same MM as 1870s - 1880s, possibly from Union Nickel Plating Works (not ref'd yet). Found on BLM limits of Marysville, Montana on "picnic hill" above a Chinese District that was pretty much gone by the 1900s w/o any later deposition-lag dumps sites.