I don't know what method was used on this piece, but typically one would coat the metal in wax and then draw the figures through the wax with some sort of stylus. You then expose it to acid (battery acid will do) and the lines exposed through the wax get etched in by the acid. You can of course carve directly into brass with a steel tool, but that takes great skill. With this piece I suspect both methods were used. The woman is lightly etched in as if by the acid technique, but the border work is much deeper and was probably carved in with a steel tool.
I'll opt for the steel tool only. Considering this was made by a true artist , I don't beleive he needed articficial enhancement tools used by souvenir sellers. This one is probably unique, unlike some drawings like "souvenir from Verdun" with a date or two , a flower here and there and voila.
This piece of art was sold on Ebay today for about 100 euros. I almost made a bid on it, but thought twice.