This isn't really a peripheral...  Many compact Macs came standard with a lock roughly dead-center on the back of the case.  The icon next to the slot is of two links in a chain.

This is a really bizarre starting point for me since I have tried to research this...  Is there a term of art other than security lock?

Follow-ups: 1) What precisely are they?  Are they solely like a tab to hook a lock onto, or do they actually lock the computer?

2) How are they removed?  Do they require specialized tools?

I realize this is not the most helpful start.  I will send a picture when possible if this isn't clear enough.