Who replaced the Halmos solid QED symbol $\blacksquare$ with the open square $\square$? In his 1955 "general topology" book, page vi, John Kelley attributed the original solid square to Paul Halmos.
The end of each proof is signalized by $\blacksquare$. This notation is also due to Halmos.
To be precise, Kelley used a solid rectangle, about 7 points height above the line, 2 points depth below the line, and a width of 3.2 points. But I can't find such a solid rectangle in this TeX symbol manual for use in Mathjax: http://www.math.harvard.edu/texman/node21.html.
In 1958, Howard Eves in "Foundations and fundamental concepts of mathematics", page 149, wrote:
The modern symbol $\square$, suggested by Paul R. Halmos, or some variant of it, is frequently used to signal the end of a proof.
Even though Eves used the open square, he didn't attribute the change from solid to open to any particular author. The other books where I have seen any historical comments at all on this subject all simply refer to Halmos. No one gives any credit for who used the open square first.
Q1. Does anyone have any references which use the open square earlier than 1958?
Q2. Does anyone know of a definitive attribution for the open square?
Q3. Is it possible that the open square came first, before the solid square?
As you would expect, there are several mentions in wikipedia, but none of them definitive. The closest to an explanation is this apparently self-contradictory page: https://en.wikipedia.org/wiki/Tombstone_(typography). I don't have full confidence in the accuracy of the Halmos quote on that page. There's also Q.E.D., Mathematical proof and List of mathematical symbols.