One reason why bronze needs less physical support than marble is the tensile strength of the material. The malleability of the bronze makes it less brittle than stone. The Dancing Faun from Pompeii is a good illustration of a smaller than life sized bronze which requires no support due to its tensile strength and the fact that it is hollow which puts less weight stress upon the projections. There is no physical reason why The Boy From Marathon would require a supporting structure. It is only about one foot taller than The Faun. I think it unlikely that there ever was one. The Dancing Faun has survived since the first century. I own a life size copy and it has survived my house and garden for many years. Our eyes have become accustomed to the practical supports in marble sculpture. They are not necessary in bronze.