It’s a coaxial grounding block of the type cable TV companies use at service entrances.
Well, actually it’s not: the ones the cable companies use are made by reputable companies and work properly. This one is made by some anonymous factory in China and fails to grip the center conductor of the coax properly, causing intermittent connections.
This is something I wasted about two hours on this afternoon (I blamed myself for failing to install the F connectors on the coax properly at first, and only gave up on this theory after the connection was still intermittent even after my third such attempt).
Eventually, such repeated failures, combined with the fact that I’m no novice on installing RF connectors on coax, and combined with how the item was discounted as a discontinued item (no returns accepted) made me suspect the guilty party. Apparently, I am not the first to have trouble with this item, and the store doubtless got tired of the endless string of returns on it and decided to discontinue stocking the part.
Thankfully, I seem to have resolved the issue without an extra trip to the hardware store for a properly-made grounding block. (I shimmed the faulty center terminal with some bits of thin copper wire.) But it’s a lesson that sometimes what appears to be a good deal (I was pleased at the price discount) is too good to be true.