Joe,
It aint Bergbau. Just checked my files and the Bergbau eagle carries a shield over the lightning bolts.

About it having a swas at its chest; The picture isn;t that clear on my screen either however I assume it is there because I have never seen this eagle design without it.

Only metal insignia with this eagle that I know of are the Feldj�gerkorps Police starburst insignia (has this eagle in the center) and the items you mention (clamshell and cartouche).

Anyone have period photographic or regulatory evidence of the use of this eagle in the early days?