If it were larger, I would say that it was broken off a top of an iron fence post, specifically the decorative fence (cast iron with bronze eagles on wreaths) along the Moika Canal Embankment which separates the Mikhailovsky Zamok and the Summer Garden right at the bridge over the Fontanka Canal to ulitsa Pestelya. There are other places where this type of Imperial Eagle finial may be seen such as on the lamp posts along the the larger bridge entrance to the SS. Peter & Paul Fortress.