The only difference between the OIII with the red passband and the one without *is the existance of that red passband*.
If one looks through OIII filter without the red leak keeping the filter close enough to the eye so that no stray light is seen, one will see something like this:
well... not exactly... (sometimes there still are hints of colors, try to believe). BTW note how the rainbow colors of the word "Astronomik" become different monochromatic shades not related with the original color.
If one keeps the filter far enough that stray light from the surrondings is seen the colors "magically apperas". Colors are caused by the mechanism that compensates color dominants (it is the same causing the illusions posted in the past).
The OIII with the red leak lets the red light of the stars "leak" into the field of view. Something might happen similar to the stray light entering in the second image. That might be a possible explanation. I do not know whether it is correct or if there is some other explanation. The fact that this witness apparently conflicts with the theory (proven by a large number of serious and controlled experiments), is not enough to convince me to thow all that serious work to the trash. I bet there is some explanation not needing to question the state of the art.
Of course if the red were seen in some strong Ha regions, strong enough to stimulate L-cones, that would not conflict with the state of the art.
The correct test will be Ha. If the red is claimed in regions that can be seen in Ha then it might be real. Is the fringe of NGC1535 seen in Ha? Which parts of M42 are seen in Ha? So far I know for sure that the huygenian region is seen in Ha (but it is greener).
Suppose one sees: 1) red with the OIII with red leak, 2) not red with the OIII without the leak and 3) you do not see anything at all with Ha; what would be the conclusions?