That part is called the inner bezel (service part #922-4257), however it wasn't typically something that ever needed replacing. All replacement service parts for the G3 iMacs have been discontinued for some time now. If you managed to find a donor iMac from which to remove the part, you have to use caution releasing (prying open) the locking tabs along the side edges of the outer bezel. If you break them off, you lose the seamless, tight fit. From your photo, it's obvious that two of them popped open with the break. The plastics become brittle with age and flexing too much can cause breakage. Did you file a damage claim with USPS? Does the iMac work, in spite of the cosmetic damage? Keep in mind that those convection-cooled iMacs often developed heat-related failure of the P-A-V (Power-Analog-Video) board. There aren't any new ones available, so replacement - if done - is accomplished with a used part, having an unknown amount of remaining service life. To prevent serious electrical shock/electrocution, replacement requires discharging the high voltage from the CRT first, and should only be attempted by someone familiar with CRT-type TV repair. That's why many e-cycled those iMacs, when that part went bad. If you can mount a 120mm cooling fan above the ventilation holes near the handle on the rear housing, you may be able to extend the service life of the iMac.