It's an odd beast, not sure what it is but you might check the irrigation or agricultural sprayer guys, they use some fittings that look a little like that.
could you cut it off and use the male threads in behind - looks like they are part of a bulk tank fitting, would a female coupler thread onto that?
Check out this fitting at Graingers web site.
BONUS! :-) I crawled around inside of the tank and found that on the inside of this adapter there was a 2 inch PVC pipe. one $1.20 cap later and I am able to block this port. Love it when a simple solution appears.
SOB fitting. None of these SOB have a adapter.
Hang a belt strap wrench over outside fitting apply force (not to MUCH) counterclock wise.
Remove SOB fitting discard. Use standard NT National Tread that it is attached to (pictured).
IF IF IF fitting does not move (Stuck, afraid you are going to break the tank) Take hack saw cut from convenient edge to the Treads Stop Just short of the threads. A little bit into thread no a biggy but try not to. Repeat try to remove.
IF IF you do not have a strap wrench put two bolts in the wholes of SOB fitting Use stick pipe across bolts to loosen.