A few weeks ago I received unique pictures of the Ericofon, showing the heart of the phone.

Copyright: Thomas Becker, Hochschule der Künste Bern (HKB), Konservierung und Restaurierung (KuR), Switzerland

Thank you Thomas!!

Update 03/05/2019:

If you look at the right picture above and the last picture in the Sewn Open  post, you can see that the bar is obviously a ‘standard’ that occurs more often in flat earpieces. But still no clue why…