There was a little flat plate (I've got one laying around somewhere), and they'd put a red gel filter in it. Acted like a safelight when they were
using ortho graphics films. I'm not implying that was the only use for that slot. If three or four-color separations were being made, other
deep gels could be used. And the waterhouse option. But there were also specialized versions of these which have not only the typical multi
bladed aperture, but a second aperture which produced an adjustable square hole!