not too many years ago a young Master Plumber sent me on a wild goose chase to get a 5/8" (or was it 3/8"?) threaded fitting... just like the one in that picture above. I got one. Hours later I called the manufacturer of the device that the young plumber was going to connect; they told me it was 1/2" NPT. That explained the confusion. Before going out to get the "right" fitting I had asked the plumber to show me why we needed it, and he showed me but I cannot remember how it came to be that he drew that conclusion, but he did show me that one thing or another didn't fit on it... I wish I could remember now, just for my own curiosity.
So maybe you have been through the same process and come to the same conclusion. Have someone else take another look, to see how they would get a more standard size to thread onto it...
david