It reminds me of the black plastic adapter that Antenna Specialists sold to adapt their 3/4" NMO antennas to the 3/8" snap in mounts. The difference is the Antenna Specialists adapters had a small metal "plug" in the center where the coax center conductor would be wrapped around. I could almost see the small metal center plug (removable) being replaced by that large threaded assembly sticking up in the photo, but that's using my imagination.
Dewey
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