Hello
Look:
http://www.925-1000.com/americansilver_U.html" onclick="window.open(this.href);return false;
«Art Nouveau» style - specially its well raised-relief - and visible part of maker's mark I guess its unique producer could be only «Unger Brothers». They were known for her famous «Art Nouveau» product range. Herewith you've also an indication of objects time range to be produced.
Why it's marked as it is? Brush body's rim is hollow below mark's space! It would "destroy" the piece if puncher would punch "as usual". If you look now in «925-1000» how letters combination of «UB» should be - and then examinee yours «UB» (for "little" products) I think you would guess too: Unger Brothers.
Kind regards silverport
.