This small ledge allows you to use Nikon G lenses on older, unsupported cameras. G lenses are lenses without an aperture ring, the camera needs to control the aperture using an aperture lever, usually requiring electronics.
You can read more about Nikon lenses here: https://kenrockwell.com/nikon/compatibility-lens.htm

Older cameras without any electronics therefore cannot control the aperture of these lenses, they will always be stuck at the smallest aperture, which is unusable in most cases.
This 3d printable ledge can be inserted into the aperture control of the lens, effectively letting you control the size of the aperture by using different lengths of ledges.

You might ask yourself, why not just use older lenses which have aperture rings?!
Lenses, especially in the wide angle spectrum, have made great advances in the last decades. Some focal lengths are simply not available in the same quality (distortion) with older lenses. Additionally they might be collectable with a steep price tag (like the older Nikon fisheyes).

To use the ledge you need to move the aperture lever of the lens and insert the ledge (the lever has a spring, by inserting the ledge we are forcing the aperture to stay open and block the spring). The space where the ledge needs to fit into is different in different lenses, you will therefore have to try out multiple of the different sizes provided, or edit the open-scad file. I provided a simple width parameter which you can modify to your liking.
I have also provided a collection of multiple different sizes as STL which worked for some of my lenses, there is also an STL which contains sizes from 2mm to 10mm, printable in one go for your convenience.

I have tested these on an Irix 15mm f2.4 and a Sigma 50mm f1.4 using a Nikon FM. I printed using an Elegoo Neptune 4 in PETG, tested in PLA as well.
Keep in mind that these are quite small prints, they don't need to be high detail but still, make sure your printer is calibrated well.

To achieve f2.4 on the Irix 15mm I used a 7mm ledge,
for the Sigma 50mm at f1.4 I used a 5mm ledge.
To achieve smaller apertures simply insert a smaller ledge, it will be up to you to measure the resulting f-number, you could figure it out using a light meter and compare against other lenses.

To make sure that the ledge works as expected you can use the aperture preview on your camera. This is also what you will need to do to use your light meter, since G lenses do not have the aperture ring to tell your camera which aperture is set.
Press the aperture preview, adjust your settings accordingly, let go of aperture preview and shoot!