Mortise Housing (without core)
The default for these housings is an opening which accepts Small Format Interchangeable Cores (SFIC). When in doubt, this will be good for you 90% of the time, because SFIC from all brands are the same size. That said, if you have Full Size Interchangeable Cores (FSIC) you'll need to select the specific corresponding brand because each brand's FSIC are different sizes.
The cam / tailpiece you'll get will need to fit your door hardware. If you're not sure which type you need, email us at firstname.lastname@example.org and we'll help you find out. Or, write us a note in the comments box and we can even provide the two most common cams so you can install the one which works.