Canon has been a very popular camera brand for a very long time, making a lots of cameras and lenses with various mounts. In 1964, Canon introduced the FL mount for 135 film SLRs, an exceptionally well-engineered mount using a wear-free breech lock. In 1971, they upgraded that to FD mount, which basically added camera coupling for automatic aperture control. The catch is that changing lenses is slower and more awkward than using bayonet mounts... so, in 1978, Canon made the disturbingly clever "new FD" bayonet that (using the opposite gender of other camera bayonets) is compatible with FL/FD. This last variant is commonly known as FDn or sometimes nFD. Lots of lenses were made in these mounts, but their very short flange distance meant that they were orphaned when Canon switched to their current, incompatible, EF autofocus mount -- at least until mirrorless bodies with short flange distances came along, and now there are lots of commercial adapters.
Ok, so why 3D print this one? Because we can seems like a decent answer -- this is an easy print in 1 piece, although it does require a little support for printing. It will work fine on manual M mounts, but the big problem for most FL/FD/FDn adapters is use on a Techart Pro LM-EA7, which allows the latest Sony E mount bodies to autofocus most lenses that can be adapted to M mount. FD/FDn lenses do not behave as manual aperture control lenses by default, but typical adapters cannot mount on an LM-EA7 because the open/close rings hit the motor housing. This adapter fits because it uses an internal stop-down post. There is also the fact that the LM-EA7 adapter has a 700g weight drive limit, so we don't want to waste weight on a heavy adapter (this one weighs about 7g).
The last two photos were taken using the LM-EA7 on a Sony A7II, respectively using the Canon FL 55mm f/1.2 shown and a Canon FDn 135mm f/3.5, both wide open.
There are now two versions posted; the newer one is slightly improved. Either one is a very straightforward print -- once you enable support to build platform for the stop-down post. I used "dishwasher safe" high-temp black PLA. This is a pretty thin-walled adapter, so make sure that you use filament that has good opacity. It should be a little tight, so work a lens in the mount a bit before attaching to a camera -- that way, you can clean any little shards that the flange might cut the first time you fit a lens to it.
Note: printing with other materials, I've noticed that the thin walls tend to deform a little. Strangely, nothing seems to help -- except printing on blue tape! I don't know why. However, if you have problems with the thin FD mount part deforming a bit, try printing on blue tape.
The M side of the adapter has a dot in the bar-patterned grip portion of the adapter that you align with the M locking pin for mounting and then turn clockwise to lock. To unmount, you simply depress the M mount release lever/tab on the body (or LM-EA7) and turn counter-clockwise, lifting the adapter out when the dot aligns.
The FL/FD/FDn side of the mount is marked "Canon" on the bottom and "FD" on the top. It is marked "FD" instead of "FL" because it has a mechanism for stopping down the lens, so it will work with FL, FD, and FDn lenses. However, there's a slight trick involved, which is why I've posted this video. The stop-down post inside the adapter doesn't move. Instead, partially mount the lens slightly rotated to get the post contacting the correct side of the lens stop-down pin (or tab), then twist the mount to the correct position and complete pushing the lens and adapter together. It should look as seen in the second photo. For FL or FD lenses, you then lock by twisting the breech collar as you would for any other FL/FD mount. For FDn lenses, you simply twist the entire lens on to lock.
To remove an FL/FD lens, simply turn the breech lock ring the opposite direction and pull the lens straight out of the adapter. To unmount an FDn lens, find and depress the unlock button on the lens, then turn the entire lens while depressing that button. The FDn lens will lift out when you have rotated it to the correct position.