I have one from Amazon too. I am not sure the brand or details, but it looks like the one Buckeye Guy links to.
I made up a heavy wall 2" square tube 6' long, with a mule hook on the far end, to go into the receiver as a boom pole, and this thing also serves me as a beam between tall jack stands for lifting things.
But my primary use of this adapter is for putting 3PT attachments out front. Most useful is the landscape rake (mine does not have wheels) for reaching into tight corners and dragging debris out. Also useful is my regular 3PT boom pole, which reaches further than the homemade one above. I may have tried my subsoiler/ripped once or twice for pulling tree roots, which can pull sideways harder than I can by myself, but of course it doesn't have much lifting capacity this way.
The two lower attachments for holding the 3PT pins slide freely on the square tubing. No, they do not lock into place. A significant limitation is that they have quite a lot of play so the attachment has a very sloppy fit and can move around a whole lot. This can be a help, though, for the rake, because it's a bit like a float function but it doesn't have the considerable weight of the loader arms and the adapter, which would drive the rake a bit too hard into the ground.