It was an accessory originally intended for the Macintosh 128k but which could also be used on the Mac 512k through the Plus.

http://www.vectronicsappleworld.com/macintosh/articlepics/macmanual/image19.jpg

http://homepage2.nifty.com/56thWAREHOUSE/SYUHEN/COMPACT.html

And in light of how high some EBAY auctions are for these old Macs, such a security cable would make practical use even today!