Two different kinds of DVD-RAM drives were used in the Power Mac G4 (AGP) computers. They have the same specifications and are functionally similar, but were made by different manufacturers. Because these drives were provided by different vendors, there are some differences. The main differences between these two drives are the tray (the slot you put a disc into) and their ability to accept discs of nonstandard size and shape.
One of the DVD-RAM drives uses a slotted tray, shown in Figure 1 below. This drive accepts only discs of a standard size and shape.
Figure 1: Slotted tray
The other DVD-RAM drive (shown in Figure 2 below) uses a flat carrier tray. In addition to standard discs, it can accept discs of a nonstandard size and shape.
Figure 2: Flat carrier tray
For more information on the size and shape of discs that can be used in your DVD-RAM drive, please see article 58641: "Macintosh: Using Nonstandard Discs in Optical Media Drives"