FIC/DSM-Perf injectors strange size?

[95GST]

I am installing my FIC injectors and I was wondering
they seem a little shorter than denso, it already come with a "grooved" ring on both ends. Seems to plug right into the rail

But the head part, I don't understand, do you still need to put on the stock thick O-ring? Or do you just install the injector the way they came?

 

[L2RTSiAWD]

You don't need an O-ring for the head since it's already attached to the injector itself.