Hi everyone. Im just getting back into the darkroom after about 5 or 6 years. Dusting off the equipment and getting everything in order. I came across a part that I'm not really sure what it's for. It looks like it's for underneath the enlarging lens where the red filter goes. In other words it looks as if it's interchangeable with it. It is square, plastic, with a round thin plastic lens at the center. It's sort of opaque. Light will pass through it but you can't see clearly through it. I have two enlargers. One set up for B&W and one for color. If anyone has an idea of what this may be for I'd appreciate it.