After first visual and feel inspection I didn't find the o-ring either and emailed Corwin Arms about it. Prior to their response I put on my reading glasses with a strong flashlight and was able to see the o-ring at approx mid-point recessed into the barrel lug.
I was going upload a photo but for some reason this is the only site I've been on that doesn't allow you upload a photo from your own computer. Very strange.
You were right. There is one recessed in there between where the ports are and the outer lip. It doesn't protrude out at all and I over looked it. Thanks for the info. I was wondering how it worked so well without an O ring