There was not a M533i, but there are M535i, it was a styling option for the e12 535i and the e28 535i. Go to this link
www.m535i.org, every question you have will be answered there, also try
www.mye28.com, they are the site for the older 5 series. I have an 86 M535i, euro model, gray market. Originally about 600 dealer imported cars,less than 1/2 that still around. They are real M535i's, not dealer installed options, tag over passenger strut tower should say M535i on it as well as a tag in the drivers side B pillar. If you email me I will send pics of mine. These cars were the basis of the M5, and later grew to be the M5.
HTH, I post on mye28 as BNC...hope to see you there....