Totally agree with Tony:
Two ways to "cast" the bronze of this M42 eagle;
---1---Bronze cast in a female mold sculpted in hollow, then compressed by a male mold (in relief). In this case, the obverse is clear, and the reverse a little more blurred, as the "male" press is removed before the metal cools, which cools in the female mold (obverse side).
---2--- Bronze cast in a female mold carved in hollow. The bronze spreads out in the recessed mold and cools. The reverse side (fixing side) is not pressed, and roughly follows the relief of the bottom mold.
In both cases, the thickness of the metal may vary slightly, unlike stamped brass sheet, which always retains the same thickness.