The SAM4s SPS-300 series and ER-900 series both use this set of two journal spool locks. These locks are what holds the journal take up spool in place while it winds the journal paper up. Very rare to have to replace these parts and I would suspect if you are that someone broke them off being way too rough with the register. These will not break under normal use.
This should be a simple repair and there is no need to unplug your register while replacing. Hopefully no one will break them off again! This is a set of two locks. Of course they are only for the 300/900 models with take up spools.