Here are some more views of the device.
It almost looks as if this is an after market item. When it is pushed in, it closes off the light source, or with the paper covering removed would be a filter. You can see that the protective paper cover has never been removed. I think the plexiglass is red, but have not peeled the paper back.
The knob design on the tray doesn't match any other knobs on the enlarger.
I have two other, newer 23C2 heads and the lower bellows on these are glued down. On this older head, it is screwed to the casting that holds the lens board. It has the regular compartment for ~5.5"X5.5" filters, but no swinging tray.
The last image is of a newer housing with the swing away filter tray.