As it turns out...what I thought was a machined surface that looked like an o-ring, turned out to be an actual o-ring. It is a sealing washer. AS you know, the adapter plate has a gasket where it bolts to the NP203 xfer case. Those bolts, if not sealed will allow fluid from the xfer case to leak. Turns out, nothing special. just a bolt with a sealing o-ring - that can be replaced. I will install new orings and sealant when I mount that adapter plate to xmission.
I found this exloded view on another forum I was browsing... pay attention to item 7.