No, this is a spacer ring, not the slip-on ring in which the AN glass is mounted. I do have the AN glass, too. It was packed separately for it's protection during shipping. I was examining the enlarger as I was unpacking and reassembling it and had not yet installed the AN glass.
What I'm told by someone else is that the spacer is used with the AN glass. But it does not go between the condenser and the AN glass; it goes where I found it, inside the condenser housing. There is a shoulder on the condenser shell that rests on the spacer. The condenser in its closed position is then raised by the thickness of the spacer, 3mm. This is to allow clearance below the condenser for the AN glass. The AN glass is usable without the spacer, but without the spacer there's less clearence when the condenser is open. At least that's my understanding. And it seems to make sense.