EoTech ?
I have the 556 and I think one of the main differences with the different replica units are which battery they use.
I have a few devices using CR123 Lithium so standardised as it makes carrying spares in the field easier.
After I bought one, I found that the replicas aren't truly holographic, which the real ones are and which is their real benefit.
I also bought the plastic insert to protect the lens and found this didn't fit properly.
So, I ended up with a magnifier and flip up lens protector on my rail as well.
There are a few US military documents floating around the net covering infantry tactics and a few cheapish books on Kindle.