HP Color Laserjet CP3505n

[Bridporter]

 I notice that when I removed the Black toner cartridge a small plastic hinged piece dropped down in the path for reinstalling the cartridge. This will not retract upwards and, as a result, the cartridge will not fully go back in, stopping front panel from closing. Any clues what this is and how I cure it? The other three cartridges don't seem to have this piece.

[Valsimot]

Toner lock, that printer model is famous for the poor engineering of that mechanism, it breaks too easily.

When the toner is removed can you lift it up with some thin object like a long needle and then insert the toner fully in? 

[Bridporter]

Is it safe to just remove this piece of plastic? I can't see what would make it retract to allow the black cart to be fully inserted. Is there some sort of interconnect with the front door that should make it lift when the door is open?

[Valsimot]

It locks the toner cartridge into the position. There are two possible options it will work just fine with a broken lock or it will give you an error message Toner not engaged.
Try to insert a cartridge to see if will it work. You have locks on both the left and right sides, if the one on the left side is broken it will probably work without it.