It's definitely a 98 head. 98-04 Frontier/Xterra KA's are probably the same way.

The cam plug thing: See that oil hole on the cam in the last pic? That's to lubricate the journal that isn't there on the 98 head. There is a seal in the back of the cam. It's like a stopper in a bottle. You need to push it into the cam farther to block off that last oil hole. If you don't, oil will come flying out of there unrestricted, possibly causing low oil pressure.