First of all I'm new here. After reading this my head is fine but my eyes going blind. I got two theories. The first is that "Beware of the 6" has dual meaning. beware= 6 letters. 6 of 6 meaning that six things or events will take place.
1.helhounds are unleashed by round 6(mostly before that)
2. nova 6 gas crawlers that also affect the nazi zombies
3. 6 mystery boxes/ teddy bear
4. the 6th dismesion (don't correct me) time zone thingy
5. the rooms you teleport to
6. the six maps
The 2nd theories: maybe if we look at "Five", pentagon= 5 sides but perhaps there a timeline for the maps. And if you take off the dot on the "!" you get 1: "Beware of the 61" (Area 61)
p.s. I just threw that last one in there because of those shards.