I looked for brass flag stake and brass umbrella stake and there are some that are close but not exact. There was also one called urban clothesline.?They show wall mounted also so you have to scroll through them all.It could also be for badminton net?
-This is an older thread, but I'm reviving it to see if there any new thoughts on it... I'd swear there was a thread with one of these that got solved, but I can't find it.
It's definitely buggy/carriage hardware, from the end of a harness pole. A leather strap or reins passes through the rectangular opening. Might be a variety of Whiffletree-hook... the Hawley's Patent whiffletree-hook shown in the early-1900s horse-harness catalog scan (see below) looks very similar.