"Security system" on 1983 300SD and key codes
 

Ordered a replacement tumbler and spare key for my '83 last week and they came in today. The existing tumbler was non-factory, and I wanted to stop having a two different keys for the ignition and doors.

The tumbler, and the key it came with, match the key I have for the doors and trunk. The second key I ordered, despite promising to be matched, is completely different and doesn't work in anything.

The only explanation I've gotten from the dealership is that it has something to do with the presence or absence of a "security system". When I was ordering the key, they asked if I had this, and I had no idea what they meant, so I said no. The question never came up when ordering keys for the '84 300SD.

Anyone have knowledge of what that "security system" might be?
Or how the key codes work? The only hint I have is an eight-digit number on the second key, that doesn't seem to have an equivalent on the tumbler.

Its for the alarm, your cars don't have it they are to old. Mine does, damn near doubles the cost of the tumbler.

The original keys for the alarm had the "red dot", that was put their for marketing.