How To Repin A Lock 

Wow, that's great! I've also used a rolled up bill and a pen shaft for a plug follower on a pinch

Christine only populated 5 of the 6 chambers with springs and driver pins. (she did say she was doing six, so good catch). It would be bad to have more springs/drivers than key pins on your lock because the drivers and spring would fall into the cylinder chamber and the spring would get dragged the next time the lock was opened jamming the core. (I've done this and it take a good amount of force to push a jammed core out of the body)

So glad our video was helpful! It is hard to tell without seeing, but if one key works and others do not (when they all originally worked), it sounds like this piece may have been a part of a master key system that allows for someone like a building owner to open all units without each unit's key opening the others.

@@LockpickExtreme sorry let me clarify. I mean to say everything seems to be functioning as expected. Random keys can't open the doors but the keys that are meant to can. The lock can't be pushed in manually, and the latch works properly yet i have this extra piece lol. I thought the pin went in with the retainer cap pin but the retainer cap pin wouldn't fit properly with it and the spring in there. That's the only place I could see that this particular pin could go in. Literally the only other place i could fit this particular pin would be inside the keyhole itself. It's about twice as long as the key pins and about as thin as the center of the spool style pins

That will work if you know what pin lengths you put into each chamber and can cut a key to code (pin length numbers). If you want to use a random key that fits your keyway, you can pin it with the key in the cylinder and drop in pins until they're flush with the top of the cylinder.

Yes, different manufacturers have different pin lengths (corresponding to the depth cuts on the key). Some lock brands are just white label names of standard manufactures, like the Home Depo Defiant locks are all Kwikset specifications for their locks, keyways and pins.