Ok, I just want to leave a quick and final update on this subject. I believe it's important for people like myself trying to revive their vintage iMacs today.
So, the iMac G3 until 1999 (tray loading) and the iMac G3 launched after 1999 (slot loading) have completely different front outer bezels. At a quick glance they look the same but if you carefully study them you'll find that the older model top part is bigger/deeper and the lower end over the Optical drive is straighter than in the newer model. Also, the internal tabs and screw holes are in completely different places and the apple logos are slightly different in size.
Unfortunately I've learned this the hard way.