What's the difference? ZK7 unloader or K7 ?

[Chuck Bergman PC 18]

I bought a ZK7 aprox 4-6 gpm unloader and I already had 2 K7 unloaders new too. I can't tell which is the ZK7 ? They all say K7

[Beth n Rod 1,279]

Hmmnn,
I checked the PDF's of both and couldn't find much distinctive difference between a zk7 and k7 other than the flow/pressure
ratings and the labeling.
Any techies got an insight on this?

Rod