I'm not sure I want to spin that nipple off... seems like it should be part of what holds the sections together. At the bottom of the picture you can see what the other side looks like. The bushing is somehow attached to the nut on the opposite side, compressing the four sections together.
Also, it's been on there for 100 years, and the nipple might crush easily. I'd be screwed then...
So if I could make up to it without removing it, that would be best. I want to find the other half of that steam coupling, two of them, one for the top and one for the bottom, then I can go from iron pipe to pex.