I printed mass distributed comic books 40 years ago, and of course I have published many books. I also have talked to people involved in printing movie posters.
I agree that calling these "Printer's Test Posters" is more accurate than "Printer's Proof Posters", given that they are unlike "Printer's Proofs" in many printing fields where only one is printed.
I used this term years ago because that it what they were always called, and because sometimes sets of one of each color surfaced, and those were always called "Progressive Printer's Proofs".
From now on, we will list these posters as "Printer's Test Posters", with the following description:
"Note that this is a very rare uncut printer's test poster (often referred to as a "printer's proof")! Before a poster was printed in large quantities, the printer would print a limited number of posters as "test" posters. Because they needed to be shown to people at the studio, people in the art department, the stars, etc, they would print a number of test posters and distribute them to these people. Some of them would be fully trimmed, but often they would leave the color chart on the left edge of many of the posters, which added an extra inch to the width of the poster (which explains why this poster measures 28" x 41"). Once these printer's test posters were shown to the various people in a position to "OK" them, the printer would print the full print run of the posters. Not only is a printer's test poster far more rare than a regular poster of the same type, but it is also 100% genuine as there is no fear of purchasing a reproduction!