I'm really struggling to understand this

There's a blade key which for most MBs from '98 will let you in but not start the engine since the ignition switch won't take a blade. There's a valet key nicknamed Mickey Mouse key that will let you in the car and start the engine. If this link works, I think this is what you're talking about - https://mbworld.org/forums/e-class-w210/508290-cost-flat-spare-key.html . At any rate, it talks about a master key which I guess looks like Mickey Mouse -



Sixto
83 300SD can be yours
98 E320 wagon

Sixto
83 300SD can be yours
98 E320 wagon