EUREKA!!! Woohoo, I am SO excited, I know what the problem is… 
@Faroe @Cedar @beethinking
The Flow SBB has been made with #6 Hardware cloth, not the usual #8!!! Plus, I have photographic proof! 
The size normally used for SBBs is #8 as no bees can get through it. Small bees can get through #6. In fact #6 is used in some pollen traps. I have a Bee Thinking non-Flow SBB and the mesh is DEFINITELY smaller than on the Flow SBB, as you can see in the photos. Personally, I will not use the Flow SBB now - I don’t want a bee leak. Shame, because it is a great design, except for the mesh size! 
