you should have it glass-bead blasted to remove the surface rust and that will leave a nice matte finish for bluing...
While you say that it shows no sign of grinding, the only way to be sure is to fit an upper receiver and if the upper receiver protrudes past the lower frame rails, then it may have been carefully ground before finishing... to remove the number.
My first take is that it is all you represent it to be... an un-numbered frame, and not one where the number has been removed. I believe the manufacturer is DWM... it looks closest to the milling marks shown in the thread mentioned previously above...
Just my $0.02 Spend it wisely...
