The 'Not for Ball' proof mark was used from 1875 to 1887, not 1887 to 1899, so your gun is a bit earlier than you thought. I agree with Roy Hebbes comments on source and quality. Middle to low grade Birmingham guns were often made without any name on them and a blank panel, the retailer could then order the guns with his name on them or have them 'finished' locally.
The 'Not for Ball' should be associated with a breech and muzzle measurement, eg. 12B, 14M. If this is missing, the Not for Ball may have been applied fraudulently to get the gun into the antique market and avoid restrictions on both the UK and US side of the pond.