> Do you really think they have a production line where every week or so they create a different mother board adding this circuit?

No, I do not think that. I assume they hand modify a few boards at a time, just like the NSA.

https://arstechnica.com/tech-policy/2014/05/photos-of-an-nsa...

Modifying every single board would violate one of the tenants of spying... it would make it much more likely for the implant to be detected.