what is that additional button/rod on Zenit-1

[landstrykere]

I'll just duplicate a post left on ussrphoto:

I am about buying a Zenit-1. I have the Zenit-3 and the Zenit-S, and interested to compare. Unless you have time to search on Avito while in Russia, they otherwise sell relatively expensive online outside RU. I spotted one cheap (no, not on Ebay), from a seller who has not much clues about.
The strange thing is an additional button/rod:

[landstrykere]

ok, mystery solved. I should have thought of it, but because it's on the top plate I was instead thinking about a mechanical tweak: it's an added flash connector.
i got the answer from Guido on Ussrphoto, and avatars of added flash connector on RF of the same times:

https://www.g-st.ch/privat/kameras/bilder/zorki/00021570.jpg