But I am sure I am right, so let's talk about a cast puzzle: cast oskar
This puzzle is old, not sure about the date, but just have a look at the package. There is no mention of Hanayama, the difficulty is not written, but the name beginning by "cast" is clearly shown.
So my assumption is that this is the old Hanayama company which produced some puzzles and then they changed their name and expand. My assumption is even stronger when I look at the small paper inside which shows some famous cast puzzles: cast jey, S&S, nut, star, elk.
However this oskar key has never been made by the nowadays known as the Hanayama company, not sure why, contrary to the other puzzles in the series that are still produced.
The small paper is in Japanese so I cannot comment about it. I assume that the designer is Oskar (as regards to the name of the puzzle), but maybe I am wrong?
By analyzing carefully (that I did not not at the beginning) both key you should quickly understand how it should get out of the rectangular shape...or that's what I thought. I think I can spoil a bit as I am sure not a lot of people will be able to play with it. I thought a key should get free first and then the other key would follow. But at the end it appeared that both key got free at the same time while still "attached" to each other and when both keys were out of the rectangular block then you can separate them.
This nice cast puzzle is far from being simple as you need precise moves and many moves to solve it. But it's not overly difficult, so in my opinion it's clearly worth a 4/6 for the difficulty. It's a very nice concept and definitely collectible.