If they didn't purchase Webster's old molds, they nearly completely ripped it off. I also have 3 brushes in the same pattern. When I get home I can get a close up of the Webster Co makers mark but if you zoom in you can see their mark next to the Sterling mark. Purchasing other companies old molds is not unheard of. Webster became a subsidiary of Reed & Barton in 1950 and was sold odd in 1958 to Frank Smith, and with art nouveau entirely out of vogue by then it would have made sense to sell the molds if you could, especially if the company already had a glut of patterns. I have a hairbrush in the Unger Bros "Loves Dream" Pattern but with hallmarks from Birmingham England c.1930's. The brush is not similiar, it is exactly the same as signed Unger Bros. versions. From what I have read, the Unger Bros company did sell their molds as they became unfit for their purposes.
I am not saying that it is a common thing for silver companies to do but as they are businesses I can't think why they wouldn't do it.