If with “back panel” you mean the back part of the 2-part so-called “slim cover”, the initial batch of the Fairphone 2 from the end of 2015/start of 2016 didn’t come with the slim cover, which was introduced later towards the end of 2016 because the initial 1-part cover was not standing the test of time for quite a number of users …
https://forum.fairphone.com/t/fairphone-2-production-and-delivery-thread/11149
https://forum.fairphone.com/t/celebrating-fairphone-2-launch/11475
https://forum.fairphone.com/t/new-slim-back-cover/22949
So, a Fairphone 2 could very well be initial batch, but not because any kind or part of cover is missing.