Re: What is this?
That gear reducer is a typical right angle unit which has a keyed shaft out of the side your chuck is attached to. If your setup was built/assembled by a hobbyist, the connection between chuck and reducer could be anything. The threaded version of those chucks come in two different sizes, and If you look into the bore of your chuck, you can see internal threads...but they're isn't anything threaded into them, so that's not how the chuck is apparently connected to the reducer. Your pictures show 3 socket head cap screws through the body of the chuck, and those are not typical of these small chucks. Have you tried removing them to see if the chuck comes off...my bets are that it will. Keep us posted.
Stuart
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