What is this? (strange hole on FED 1)

[thevideographer]

I just got a FED-1d on ebay because I can't afford a Leica, and I noticed this strange hole on top of the rangefinder. It's not in any pictures I've seen of other Fed, Zorki, or Leica cameras. Any ideas what it could be? Here's a photo, the weird hole is circled in red.

 

[compur]

Looks like a flash sync terminal