Need the latch(?) part that this goes into.

Thanks for writing in! I believe you may need the 10-563 Concealed Door Latch and either the 10-556 Knob Assembly or the 10-557 Lever Handle Assembly (ADA Compliant). I'm not able to see the internal parts of your latch so I can't be quite certain, so you'll need to remove them and compare the specifications (on the website) to confirm.

Please see the tutorial, tagged below, for instructions on how to remove/install the hardware.