"Don’t be deceived by its simple appearance. What looks like a strangely placed door or free-standing oddity is a gateway that leads to a hyper-dimensional machine whose single entry can be moved to any point of history in the universe. Step across its threshold and enter the impossible. Enter the door to infinity."
The Door To Infinity, a three page Who-with-the-numbers-filed-off setting for Mini Six.
Just for that, I'm going to use one of their adventure hooks. So there!