Hi,
This is a Bosch Zundmagnet or Magneto for providing the HT ignition ignition for the sparkplugs This is an Aero-engine part from an early JUMO 211 engine and it looks to have the drive adapter from the JUMO 211 engine still fitted. This Magneto would be correct for the early He 111 H type pictured in the lake.
Cheers
Eng