I thought that beginning in 1986, that a driverside airbag was standard on ALL Mercedes cars. Here is a 86' 420SEL, without airbag (link). At first, I thought maybe someone had it removed and installed a regular wheel out of a 85' older without airbag. However, I also noticed the different knee bolster that the non-airbag equipped cars have. I suppose its possible that was changed too? Then I was thinking... maybe this is a non-US model and did not come with one? I am thinking all had them though by 86'... even non-US models?

http://cgi.ebay.com/ebaymotors/1986-Mercedes-Benz-420-SEL-No-Reserve_W0QQitemZ190051396827QQihZ009QQcategoryZ6331QQssPageNameZWDVWQQrdZ1QQcmdZViewItem