hehe..I have quite a few older Macs(beige powerPC) and five G4 towers one being a dual500..and have noticed the same thing ..
Seems like at some point you needed to buy an "apple" monitor to go with the system.
If Im not mistaken I also remember that the Keyboard and mouse was a separate purchase as well..
Yes the joys of Apple.
I got my apples for free throughout the years from work. I also have a ton of apple motherboards, sonnet accelerators, cpu's etc. ug so much stuff so little time hehe
Enjoy