The Square Circle is a prop used for productions and is usually attributed to Louis Histed who popularized it with his marketed effect The Chinese Pagoda in 1930.
The magician shows a square box with a window in front and a cylinder which fits in the box. When the tube is in the box, it can be seen through the window. The cylinder is lifted out and shown empty while the black empty interior of the box can be seen through the window. Yet, when the tube is placed inside the box, the magician reaches in and produces all kinds of items.
The Davenports sold a Square Circle effect under the name of Wunda Villa (created by Eric P. Wilson).
In letter to Dariel Fitzkee in 1947, Stanley Collins also claimed to be the originator of the principle. Fitzkee asked, "Why didn't you mention it in A Conjuring Melange? You had a golden opportunity."